iio: mxs-lradc: disable only masked channels in mxs_lradc_hw_stop

Disabling of the touchscreen IRQs should be done in
mxs_lradc_disable_ts. So disable only the masked virtual channels
in mxs_lradc_hw_stop and finally remove the unused function
mxs_lradc_irq_en_mask.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>