net: broadcom: bcm4908_enet: set MTU on open & on request

Hardware comes up with default max frame size set to 1518. When using it
with switch it results in actual Ethernet MTU 1492:
1518 - 14 (Ethernet header) - 4 (Broadcom's tag) - 4 (802.1q) - 4 (FCS)

Above means hardware in its default state can't handle standard Ethernet
traffic (MTU 1500).

Define maximum possible Ethernet overhead and always set MAC max frame
length accordingly. This change fixes handling Ethernet frames of length
1506 - 1514.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>