cxgb4: Don't assume FW_PORT_CMD reply is always port info msg

The firmware can send a set of asynchronous replies through FW_PORT_CMD
with DCBX information when that's negotiated with the Link Peer. The old
code always assumed that a FW_PORT_CMD reply was always a Get Port
Information message. This change conditionalizes the code to only handle
the Get Port Information messages and throws a warning if we don't
understand what we've been given.

Also refactor t4_handle_fw_rpl() so that core functionality performed by
t4_handle_get_port_info() for a specified port.

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