iio: imu: inv_mpu6050: inv_mpu_ring: Demote seemingly unintentional kerneldoc header

This is the only use of kerneldoc in the source file and no
descriptions are provided.

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

 drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c:118: warning: Function parameter or member 'irq' not described in 'inv_mpu6050_read_fifo'
 drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c:118: warning: Function parameter or member 'p' not described in 'inv_mpu6050_read_fifo'

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>