Input: atmel_mxt_ts - convert to MT protocol B

Atmel touchscreen chips can use MT protocol B because they can assign
unique id to ABS_MT_TRACKING_ID from finger id provided by hardware.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>