iio: mxs-lradc: move STMP reset out of ADC init

This patch moves the STMP reset out of ADC init function so as to remove
the possiblity of an error return which will be necessary for PM ops support
patches to follow.

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