bonding: allow all slave speeds

No need to check for 10, 100, 1000, 10000 explicitly. Just make this
generic and check for invalid values only (similar check is in ethtool
userspace app). This enables correct speed handling for slave devices
with "nonstandard" speeds.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Reviewed-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>