sfc: Remove fallback for invalid permanent MAC address

By the time we look at the MAC address in efx_probe_port(), either the
driver or the firmware has already validated the board configuration.
The possibility of having an invalid MAC address just isn't worth
considering.  It certainly isn't worth having a compile-time option
for this.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>