bcm63xx_enet: remove pointless mac_id check

Enabling the ephy clock for mac 1 is harmless, and the actual usage of
the ephy is not restricted to mac 0, so we might as well remove the
check.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>