ibcore: Fix endless loop in searching for matching VLAN device

In r337943 ifnet's if_pcp was set to the PCP value in use
instead of IFNET_PCP_NONE.
Current ibcore code assumes that if_pcp is IFNET_PCP_NONE with
VLAN interfaces so it can identify prio-tagged traffic.
Fix that by explicitly verifying that that the if_type is IFT_ETHER
and not IFT_L2VLAN.

MFC after: 3 days
Approved by: re (Marius), hselasky (mentor), kib (mentor)
Sponsored by: Mellanox Technologies