Input: synaptics-rmi4 - add sysfs attribute update_fw_status

The attribute returns the percentage complete. If the firmware update fails, it
reports a negative error code.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Chris Healy <cphealy@gmail.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>