HID: rmi: fallback to generic/multitouch if hid-rmi is not built

Commit 279967a6 ("HID: rmi: Handle all Synaptics touchpads using hid-rmi")
unconditionally switches over handling of all Synaptics touchpads to hid-rmi
(to make use of extended features of the HW); in case CONFIG_HID_RMI is
disabled though this renders the touchpad unusable, as the

	HID_DEVICE(HID_BUS_ANY, HID_GROUP_RMI, HID_ANY_ID, HID_ANY_ID)

match doesn't exist and generic/multitouch doesn't bind to it either (due
to hid group mismatch).

Fix this by switching over to hid-rmi only if it has been actually built.

Fixes: 279967a6 ("HID: rmi: Handle all Synaptics touchpads using hid-rmi")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>