Input: MT - Add flags to input_mt_init_slots()

Preparing to move more repeated code into the mt core, add a flags
argument to the input_mt_slots_init() function.

Reviewed-and-tested-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Tested-by: Ping Cheng <pingc@wacom.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>