MFC r291953:
When setting up VLANs on a Raspberry Pi ethernet port, the MTU drops
from 1500 to 1496 bytes. The MTU should remain at 1500, extending the
frame size as per IEEE 802.3. Adding IFCAP_VLAN_MTU to the
if_capabilities field in the smsc driver solves the problem. The
datasheet for the LAN9512 chip, section 3.2.3 states that the chip
supports the extended frame.

Submitted by: rpp@ci.com.au
PR: 205050