arcnet: com20020-isa: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/net/arcnet/com20020-isa.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 205:13, 203:10, 209:7, 201:11,
207:8

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>