Blackfin: bfin_mac: convert CONFIG_BFIN_MAC_RMII to CONFIG_RMII

No point in having a Blackfin-specific define "CONFIG_BFIN_MAC_RMII" that
does exactly the same thing as common "CONFIG_RMII".

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>