etherswitch: Add a new striptagingress port flag

Felix switch found in LS1028A supports stripping VLAN tag on
ingress, instead of egress. The striptag flag excepts the latter
behaviour.
Add a new flag to support the feature.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30922