iio: mxs-lradc: move TS config into suitable function

This patch moves the touchscreen type configuration into
a more suitable function. Btw this simplifies PM ops later.

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