PCI: make bitfield unsigned

Fix sparse warning:

drivers/pci/pci.h:247:25: error: dubious one-bit signed bitfield

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: linux-pci@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>