net_failover: Use netdev_features_t instead of u32

The features mask needs to be a netdev_features_t (u64) because a u32
is not big enough.

Fixes: cfc80d9a ("net: Introduce net_failover driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>