Touch toggle softkeys send a '1' while pressed and a '0' while released,
requring the kernel to keep track of wether touch should be enabled or
disabled. The code does not handle the state transitions properly,
however. If the key is pressed repeatedly, the following four states
of states are cycled through (assuming touch starts out enabled):

Press:   shared->is_touch_on => 0, SW_MUTE_DEVICE => 1
Release: shared->is_touch_on => 0, SW_MUTE_DEVICE => 1
Press:   shared->is_touch_on => 1, SW_MUTE_DEVICE => 0
Release: shared->is_touch_on => 1, SW_MUTE_DEVICE => 1

The hardware always properly enables/disables touch when the key is
pressed but applications that listen for SW_MUTE_DEVICE events to provide
feedback about the state will only ever show touch as being enabled while
the key is held, and only every-other time. This sequence occurs because
the fallthrough WACOM_HID_WD_TOUCHONOFF case is always handled, and it
uses the value of the *local* is_touch_on variable as the value to
report to userspace. The local value is equal to the shared value when
the button is pressed, but equal to zero when the button is released.

Reporting the shared value to userspace fixes this problem, but the
fallthrough case needs to update the shared value in an incompatible
way (which is why the local variable was introduced in the first place).
To work around this, we just handle both cases in a single block of code
and update the shared variable as appropriate.

Fixes: d793ff81

 ("HID: wacom: generic: support touch on/off softkey")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Aaron Skomra <aaron.skomra@wacom.com>
Tested-by: Aaron Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>