HID: wacom: Set button bits based on a new numbered_buttons

Prior to this commit, numbered button bit setting was done separately
for each device type in wacom_setup_pad_capabilities(). Here we add a
numbered_buttons property to the wacom_features struct and extract the
repeated bit setting code to a new function:
wacom_settup_numbered_buttons().

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