ixgbe: Resolve cppcheck format string warning

cppcheck warns that the format string is incorrect in the function
ixgbe_get_strings().  Since the value cannot be negative, change the
variable to unsigned which matches the format specifier.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_ethtool.c

@@ -1277,7 +1277,7 @@ static void ixgbe_get_strings(struct net_device *netdev, u32 stringset,
 			      u8 *data)
 {
 	char *p = (char *)data;
-	int i;
+	unsigned int i;
 
 	switch (stringset) {
 	case ETH_SS_TEST: