fix(igc): use ETHER_MAX_LEN for max_frame_size

[ytakano]

Description

Use ETHER_MAX_LEN for igc's max_frame_size as FreeBSD.

Related links

• FreeBSD's if_igc.c: https://github.com/freebsd/freebsd-src/blob/31407551c3d4d29905f77abddac3fd9b649e166b/sys/dev/igc/if_igc.c#L611-L612

	 * Set the frame limits assuming
	 * standard ethernet sized frames.
	 */
	scctx->isc_max_frame_size = sc->hw.mac.max_frame_size =
	    ETHERMTU + ETHER_HDR_LEN + ETHERNET_FCS_SIZE;

where ETHERMTU == 1500, ETHER_HDR_LEN == 14, and ETHERNET_FCS_SIZE == 4.

ETHER_MAX_LEN in awkernel is 1518.

How was this PR tested?

Notes for reviewers