iio: mxs-lradc: fix buffer overflow

iio: mxs-lradc: fix buffer overflow

Fixes:
drivers/staging/iio/adc/mxs-lradc.c:1556 mxs_lradc_probe() error: buffer
overflow 'iio->channels' 15 <= 15

The reported available scales for in_voltage15 were also wrong.

The realbits lookup is not necessary as all the channels of the LRADC have the
same resolution, use LRADC_RESOLUTION instead.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>