Don't touch USB controller IO registers when they are disabled

The USB "handoff" code is an early PCI quirk to make sure we own the USB
controller (as opposed to the BIOS/SMM).  But if the controller isn't
even enabled yet, don't try to access it.

Acked-by: Paul Mackerras <paulus@samba.org> (who had an alternate patch)
Signed-off-by: Linus Torvalds <torvalds@osdl.org>