netdev: bfin_mac: disable hardware checksum if writeback cache is enabled

With writeback caches, corrupted RX packets will be sent up the stack
without any error markings.

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>