cnic: Return proper error code if we fail to send netlink message

to allow iSCSI connection to fail faster instead of waiting for the
long timeout.

Update version to 2.5.6.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>