net: ethernet: Fix EAPol packet length changed wrongly

In ethernet_recv(), it will call ethernet_update_length() to check for
ipv4 or ipv6 packet if it needs to get rid of the padding in the
packets. But for EAPOL packets, no need to do this, which may modify
the packet length of EAP type packet wrongly and leads to EAPoL
packets dropped in supplicant.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>