HID: wacom: Avoid entering wacom_wac_pen_report for pad / battery

It has recently been reported that the "heartbeat" report from devices
like the 2nd-gen Intuos Pro (PTH-460, PTH-660, PTH-860) or the 2nd-gen
Bluetooth-enabled Intuos tablets (CTL-4100WL, CTL-6100WL) can cause the
driver to send a spurious BTN_TOUCH=0 once per second in the middle of
drawing. This can result in broken lines while drawing on Chrome OS.

The source of the issue has been traced back to a change which modified
the driver to only call `wacom_wac_pad_report()` once per report instead
of once per collection. As part of this change, pad-handling code was
removed from `wacom_wac_collection()` under the assumption that the
`WACOM_PEN_FIELD` and `WACOM_TOUCH_FIELD` checks would not be satisfied
when a pad or battery collection was being processed.

To be clear, the macros `WACOM_PAD_FIELD` and `WACOM_PEN_FIELD` do not
currently check exclusive conditions. In fact, most "pad" fields will
also appear to be "pen" fields simply due to their presence inside of
a Digitizer application collection. Because of this, the removal of
the check from `wacom_wac_collection()` just causes pad / battery
collections to instead trigger a call to `wacom_wac_pen_report()`
instead. The pen report function in turn resets the tip switch state
just prior to exiting, resulting in the observed BTN_TOUCH=0 symptom.

To correct this, we restore a version of the `WACOM_PAD_FIELD` check
in `wacom_wac_collection()` and return early. This effectively prevents
pad / battery collections from being reported until the very end of the
report as originally intended.

Fixes: d4b8efeb ("HID: wacom: generic: Correct pad syncing")
Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Tested-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>