igc: return an error if the mac type is unknown in igc_ptp_systim_to_hwtstamp()

clang static analysis reports
drivers/net/ethernet/intel/igc/igc_ptp.c:673:3: warning: The left operand of
'+' is a garbage value [core.UndefinedBinaryOperatorResult]
ktime_add_ns(shhwtstamps.hwtstamp, adjust);
^ ~~~~~~~~~~~~~~~~~~~~

igc_ptp_systim_to_hwtstamp() silently returns without setting the hwtstamp
if the mac type is unknown. This should be treated as an error.

Fixes: 81b055205e8b ("igc: Add support for RX timestamping")
Signed-off-by: Tom Rix <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: Sasha Neftin <[e-mail]>
Tested-by: Naama Meir <[e-mail]>
Signed-off-by: Tony Nguyen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/intel/igc/igc_ptp.c

···
  *
  * We need to convert the system time value stored in the RX/TXSTMP registers
  * into a hwtstamp which can be used by the upper level timestamping functions.
+ *
+ * Returns 0 on success.
  **/
-static void igc_ptp_systim_to_hwtstamp(struct igc_adapter *adapter,
-				       struct skb_shared_hwtstamps *hwtstamps,
-				       u64 systim)
+static int igc_ptp_systim_to_hwtstamp(struct igc_adapter *adapter,
+				      struct skb_shared_hwtstamps *hwtstamps,
+				      u64 systim)
 {
 	switch (adapter->hw.mac.type) {
 	case igc_i225:
···
 						systim & 0xFFFFFFFF);
 		break;
 	default:
-		break;
+		return -EINVAL;
 	}
+	return 0;
 }
 
 /**
···
 
 	regval = rd32(IGC_TXSTMPL);
 	regval |= (u64)rd32(IGC_TXSTMPH) << 32;
-	igc_ptp_systim_to_hwtstamp(adapter, &shhwtstamps, regval);
+	if (igc_ptp_systim_to_hwtstamp(adapter, &shhwtstamps, regval))
+		return;
 
 	switch (adapter->link_speed) {
 	case SPEED_10: