MFC r310170, r310492

r310170:
[spigen] Make "data" part of spigen_transfer optional

Make st_data part of spigen_transfer optional by letting pass zero length
and NULL pointer. SPI controller drivers handle this case fine.

r310492:
[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