bnx2x: Fix vlan credit issues for VFs

Starting with commit 2dc33bbc "bnx2x: Remove the sriov VFOP mechanism",
the bnx2x started enforcing vlan credits for all vlan configurations.
This exposed 2 issues:
  - Vlan credits are not returned once a VF is removed; this causes a leak
    of credits, and eventually will lead to VFs with no vlan credits.
  - A vlan credit must be set aside for the Hypervisor to use, and should
    not be visible to the VF.

Although linux VFs at the moment do not support vlan configuration [from the
VF side] which causes them to be resilient to this sort of issue, Windows VF
over linux hypervisors might fail to load as the vlan credits become depleted.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>