sparc,leon: fix GRPCI2 device0 PCI config space access

bus=0 slot=0 (device0) was used internally by the PCI host driver
to access the PCI host controller itself, however that had the
effect that PCI device0 was never accessible, which is wrong
when the motherboard has connected PCI AD16 signal to a slot.
A special case for accessing the PCI host controller itself is
added with this patch, by setting bus to TGT.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>