[SCSI] st: fix gcc 4.4 warning

This patch fixes the GCC 4.4 warning reported by David Binderman and Sergey
Senozhatsky. The old version was working correctly but was not easy to read.
Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/st.c

@@ -2964,7 +2964,7 @@ static int st_int_ioctl(struct scsi_tape *STp, unsigned int cmd_in, unsigned lon
 			    !(STp->use_pf & PF_TESTED)) {
 				/* Try the other possible state of Page Format if not
 				   already tried */
-				STp->use_pf = !STp->use_pf | PF_TESTED;
+				STp->use_pf = (STp->use_pf ^ USE_PF) | PF_TESTED;
 				st_release_request(SRpnt);
 				SRpnt = NULL;
 				return st_int_ioctl(STp, cmd_in, arg);