ethernet/toshiba: use core min/max MTU checking

gelic_net: min_mtu 64, max_mtu 1518
- remove gelic_net_change_mtu now that it is redundant

spidernet: min_Mtu 64, max_mtu 2294
- remove spiter_net_change_mtu now that it is redundant

CC: netdev@vger.kernel.org
CC: Geoff Levand <geoff@infradead.org>
CC: Ishizaki Kou <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>