Input: resistive-adc-touch - rework mapping of channels

Instead of iterating over channels establish and use channel map to
retrieve data. As a side effect this will silence "uninitialized variable"
warnings.

Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/YLXR2brkc4H54xtK@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>