ixl(4): Fix SR-IOV panics

The hw and ifp of a vsi will be NULL if such ixl_vsi is allocated
for a VF. When ixl_reconfigure_filters called, it is trying to
access vsi->ifp and hw->mac.addr and therefore is casuing panic.

This commit add checks to determine if vsi is a VF by checking
if vsi->hw is NULL, before adding IXL_VLAN_ANY filter (which
is already in a VF vsi's filter list) and VLAN filters.

(erj's Note: The SR-IOV flows need revisiting; this will help
prevent panics for now)

Reviewed by: krzysztof.galazka@intel.com, erj@
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35649