Input: wm97xx - mark channels which need pen_down

Currently, battery drivers also use poll_sample() provided by the
wm97xx-core but this code always checks if the pen is down. Mark the
channels which really need this (i.e. for the touchscreen) with the
PEN_DOWN bit, and skip the checks otherwise. Now, the battery channels
can always be read.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>