Input: synaptics-rmi4 - prevent null pointer dereference in f30

If the platform data has f30_data.disable set, f30 in rmi_f30_config()
might be null. Prevent a kernel oops by checking for non-null f30.

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