[spigen] Fix spigen attaching as a driver for SPI devices nodes in FDT

Return BUS_PROBE_NOWILDCARD in probe method to make sure that spigen
attaches only to the device created in identify method.

Before this change spigen probe method used to return 0 which meant it
competed with other drivers to be attached to the devices created for
child nodes of SPI bus node in FDT.

Reported by: Daniel Braniss
MFC after: 1 week