netdev: bfin_mac: clear RXCKS if hardware generated checksum is not enabled

Otherwise we might be get a setting mismatch from a previous module or
bootloader and what the driver currently expects.

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>