Input: synaptics-rmi4 - remove pointer to rmi_function in f12_data

The pointer to struct rmi_function in f12_data is never set and was never
used. The fn pointer is also stored in rmi_2d_sensor which is a member of
f12_data.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>