iio: mxs-lradc: remove useless check

Checking the channel number is useless since mxs_lradc_read_raw() is called from
a controlled environment and the driver is responsible for filing the struct
iio_chan_spec.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>