netdev: bfin_mac: check for mii_bus platform data

If the platform data for the mii_bus is missing, gracefully error out
rather than deference NULL pointers.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>