HID: wacom: Refactor touch input mute checks into a common function

We perform this same set of tests to see if touch input is muted in
several places. We might as well replace these independent copies with
an inline function.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>