iio: imu: inv_mpu6050: inv_mpu_core: Demote obvious misuse of kerneldoc to standard comment blocks

No attempt has been made to document any of the demoted functions here.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:444: warning: Function parameter or member 'st' not described in 'inv_mpu6050_set_lpf_regs'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:444: warning: Function parameter or member 'val' not described in 'inv_mpu6050_set_lpf_regs'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:480: warning: Function parameter or member 'indio_dev' not described in 'inv_mpu6050_init_config'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:862: warning: Function parameter or member 'st' not described in 'inv_mpu6050_set_lpf'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:862: warning: Function parameter or member 'rate' not described in 'inv_mpu6050_set_lpf'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:893: warning: Function parameter or member 'dev' not described in 'inv_mpu6050_fifo_rate_store'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:893: warning: Function parameter or member 'attr' not described in 'inv_mpu6050_fifo_rate_store'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:893: warning: Function parameter or member 'buf' not described in 'inv_mpu6050_fifo_rate_store'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:893: warning: Function parameter or member 'count' not described in 'inv_mpu6050_fifo_rate_store'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:954: warning: Function parameter or member 'dev' not described in 'inv_fifo_rate_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:954: warning: Function parameter or member 'attr' not described in 'inv_fifo_rate_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:954: warning: Function parameter or member 'buf' not described in 'inv_fifo_rate_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:975: warning: Function parameter or member 'dev' not described in 'inv_attr_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:975: warning: Function parameter or member 'attr' not described in 'inv_attr_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:975: warning: Function parameter or member 'buf' not described in 'inv_attr_show'
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c:1282: warning: Function parameter or member 'st' not described in 'inv_check_and_setup_chip'

Cc: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>