net/cxgbe: support flow API for source MAC rewrite

Add support to rewrite Source MAC addresses. The new Source
MAC address is written into a free entry in the SMT table
and the corresponding SMT index is used by hardware to
rewrite the Source MAC address of the packets hitting the
flow.

Signed-off-by: Karra Satwik <kaara.satwik@chelsio.com>
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>