fm10k: prevent possibly uninitialized variable

If 'attr_flag < (1 << (2 * FM10K_TEST_MSG_NESTED))' is ever false, err
will be used uninitialized.
Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/fm10k/fm10k_ethtool.c

@@ -937,7 +937,7 @@ static int fm10k_mbx_test(struct fm10k_intfc *interface, u64 *data)
 	struct fm10k_mbx_info *mbx = &hw->mbx;
 	u32 attr_flag, test_msg[6];
 	unsigned long timeout;
-	int err;
+	int err = -EINVAL;
 
 	/* For now this is a VF only feature */
 	if (hw->mac.type != fm10k_mac_vf)