cxgbe: handle vlan PF restrictions

Co-Authored-by: Navdeep Parhar <np@FreeBSD.org>
MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D45428

(cherry picked from commit 1389314d53531e06c7ec02406b0addf7d77e7db7)