ice: allow host to clear administratively set VF MAC

Currently a user is not allowed to clear a VF's administratively set MAC
on the PF. Fix this by allowing an all zero MAC address via "ip link set
${pf_eth} vf ${vf_id} mac 00:00:00:00:00:00".

An example use case for this would be issuing a "virsh shutdown"
command on a VM. The call to iproute mentioned above is part of this flow.
Without this change the driver incorrectly rejects clearing the VF's
administratively set MAC and prints unhelpful log messages.

Also, improve the comments surrounding this change.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>