iio: mxs-lradc: Fix 'duplicate const' warning
The following warning is generated by sparse:
drivers/staging/iio/adc/mxs-lradc.c:118:47: warning: duplicate const
Remove the duplicate const.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>