ne2000: fix unused function warning

Geert noticed a warning on MIPS TX49xx, Atari and presuambly other
platforms when the driver is built-in but NETDEV_LEGACY_INIT is
disabled:

drivers/net/ethernet/8390/ne.c:909:20: warning: ‘ne_add_devices’ defined but not used [-Wunused-function]

Merge the two module init functions into a single one with an
IS_ENABLED() check to replace the incorrect #ifdef.

Fixes: 4228c394 ("make legacy ISA probe optional")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>