net: don't abort when receive a VLAN frame

bsd/sys/net/if_ethersubr.cc has an assert to check if
incoming frame is a VLAN frame, if so kernel will abort
there, just dropping it may be a bettter choice.

Signed-off-by: Li Yu <raise.sail@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>