make legacy ISA probe optional

There are very few ISA drivers left that rely on the static probing from
drivers/net/Space.o. Make them all select a new CONFIG_NETDEV_LEGACY_INIT
symbol, and drop the entire probe logic when that is disabled.

The 9 drivers that are called from Space.c are the same set that
calls netdev_boot_setup_check().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>