iio: mxs-lradc: Remove useless check in read_raw

The removed check in the read_raw implementation was always true,
therefore remove it. This also fixes a bug, by closely inspecting
the code, one can notice the iio_validate_scan_mask_onehot() will
always return 1 and therefore the subsequent condition will always
succeed, therefore making the mxs_lradc_read_raw() function always
return -EINVAL; .

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>