cxgb4: Add support for ndo_get_vf_config

Adds support for ndo_get_vf_config, also fill the default mac address
that will be provided to the VF by firmware, in case user doesn't
provide one. So user can get the default MAC address address also
through ndo_get_vf_config.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>