MFC r291953:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@292185 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f