PCI: fix issue with busses registering multiple times in sysfs

PCI busses can be registered multiple times, so we need to detect if we
have registered our bus structure in sysfs already.  If so, don't do it
again.

Thanks to Guennadi Liakhovetski <g.liakhovetski@gmx.de> for reporting
the problem, and to Linus for poking me to get me to believe that it was
a real problem.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>