PCI/XEN: Fix bug introduced by a recent change

This patch (as1516) fixes a bug introduced during the removal of
put_driver() and get_driver() from drivers/pci/xen-pcifront.c.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>