m68k: emu: Fix invalid free in nfeth_cleanup()

In the for loop all nfeth_dev array members should be freed, not only
the first one.  Freeing only the first array member can cause
double-free bugs and memory leaks.

Fixes: 9cd7b148

 ("m68k/atari: ARAnyM - Add support for network access")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20210705204727.10743-1-paskripkin@gmail.com
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>