HID: wacom: generic: read the number of expected touches on a per collection basis

Bluetooth connections may contain more than one set of touches,
or a partial set of touches, in one report.

Set the number of expected touches when reading a collection
instead of once per report (in the pre-report function).

Accordingly, reset the number of touches expected after each sync.

Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>