[efi] Exclude link-layer header length from MaxPacketSize

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_snp.c

 
 	mode->HwAddressSize = ll_addr_len;
 	mode->MediaHeaderSize = ll_protocol->ll_header_len;
-	mode->MaxPacketSize = netdev->max_pkt_len;
+	mode->MaxPacketSize = netdev->mtu;
 	mode->ReceiveFilterMask = ( EFI_SIMPLE_NETWORK_RECEIVE_UNICAST |
 				    EFI_SIMPLE_NETWORK_RECEIVE_MULTICAST |
 				    EFI_SIMPLE_NETWORK_RECEIVE_BROADCAST );