Input: synaptics-rmi4 - limit the range of what GPIOs are buttons

By convention the first 6 bits of F30 Ctrl 2 and 3 are used to signify
GPIOs which are connected to buttons. Additional GPIOs may be used as
input GPIOs to signal the touch controller of some event
(ie disable touchpad). These additional GPIOs may meet the criteria of
a button in rmi_f30_is_valid_button() but should not be considered
buttons. This patch limits the GPIOs which are mapped to buttons to just
the first 6.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Reported-by: Daniel Martin <consume.noise@gmail.com>
Tested-by: Daniel Martin <consume.noise@gmail.com>
Acked-By: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>