e1000: Fix vlan PCP/DEI on lem(4)

The vlan PCP and CFI/DEI were discarded when receiving vlan tagged
packets on lem(4) interfaces with vlanhwtag.

According to the 82540 SDM[1] (pg. 24), vlan tag is in the standard
format, so there's no reason to discard PCP/DEI.

[1]: http://iommu.com/datasheets/ethernet/controllers-nics/intel/e1000/pci-pci-x-family-gbe-controllers-software-dev-manual.pdf

Sponsored by: Stormshield (author)
Differential Revision: https://reviews.freebsd.org/D48987

(cherry picked from commit 4b29599fbbe33b75b7b58cfc5deea7a881e9a10e)