iio: adc: ti_am335x_adc: Remove a couple of unused 'read' variables

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

 drivers/iio/adc/ti_am335x_adc.c: In function ‘tiadc_buffer_preenable’:
 drivers/iio/adc/ti_am335x_adc.c:297:21: warning: variable ‘read’ set but not used [-Wunused-but-set-variable]
 297 | int i, fifo1count, read;
 | ^~~~
 drivers/iio/adc/ti_am335x_adc.c: In function ‘tiadc_buffer_predisable’:
 drivers/iio/adc/ti_am335x_adc.c:346:21: warning: variable ‘read’ set but not used [-Wunused-but-set-variable]
 346 | int fifo1count, i, read;
 | ^~~~

Cc: Rachna Patil <rachna@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>