HID: logitech-hidpp: separate HID++ from WTP processing

Previously wtp_raw_event would be called through
hidpp_raw_hidpp_event (for the touchpad report) and hidpp_raw_event
(for the mouse report).

This patch removes one calling surface, making a clearer distinction
between "generic HID++ processing" (matching internal reports) and
device-specific event processing.

Suggested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>