tcm_qla2xxx: Handle malformed wwn strings properly

If we make a variable an unsigned int and then expect it to be < 0 on
a bad character, we're going to have a bad time.  Fix the tcm_qla2xxx
code to actually notice if hex_to_bin() returns a negative variable.

This was detected by the compiler warning:

    scsi/qla2xxx/tcm_qla2xxx.c: In function ‘tcm_qla2xxx_npiv_extract_wwn’:
    scsi/qla2xxx/tcm_qla2xxx.c:148:3: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/scsi/qla2xxx/tcm_qla2xxx.c

@@ -137,13 +137,15 @@ static char *tcm_qla2xxx_get_fabric_name(void)
  */
 static int tcm_qla2xxx_npiv_extract_wwn(const char *ns, u64 *nm)
 {
-	unsigned int i, j, value;
+	unsigned int i, j;
 	u8 wwn[8];
 
 	memset(wwn, 0, sizeof(wwn));
 
 	/* Validate and store the new name */
 	for (i = 0, j = 0; i < 16; i++) {
+		int value;
+
 		value = hex_to_bin(*ns++);
 		if (value >= 0)
 			j = (j << 4) | value;