cxgb4/cxgb4vf: Add set VF mac address support

Add ndo_set_vf_mac support which allows to set the MAC address
for cxgb4vf interfaces from the host

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