iio: mxs-lradc: indexes are unsigned

Fix the following warning when building with W=1 option:

drivers/staging/iio/adc/mxs-lradc.c: In function 'mxs_lradc_trigger_handler':
drivers/staging/iio/adc/mxs-lradc.c:244:2: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>