Input: synaptics-rmi4 - add rmi_find_function()

If a function needs to communicate with an other, it's better to have
a way to retrieve this other.

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