sfc: Hold MAC lock for longer in efx_init_port()

Although efx_init_port() is only called at probe time and so cannot
race with port reconfiguration, most of the functions it calls can
expect to be called with the MAC lock held.

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