sfc: Merge falcon_probe_phy() into falcon_probe_port()

MAC and PHY probing are bound up together, as evidenced by the
initialisation of efx_nic::loopback_modes.  Remove the current
arbitrary separation.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>