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>

drivers/pci/pci.h

@@ -244,7 +244,7 @@ struct pci_ats {
 	int stu;	/* Smallest Translation Unit */
 	int qdep;	/* Invalidate Queue Depth */
 	int ref_cnt;	/* Physical Function reference count */
-	int is_enabled:1;	/* Enable bit is set */
+	unsigned int is_enabled:1;	/* Enable bit is set */
 };
 
 #ifdef CONFIG_PCI_IOV