HID: multitouch: simplify the application retrieval

Now that the application is simply stored in struct hid_input, we can
overwrite it in mt_input_mapping() for the faulty egalax and have a
simpler suffix processing in mt_input_configured()

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>