arcnet: com90xx: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: powerpc allyesconfig):

drivers/net/arcnet/com90xx.c: In function 'com90xx_setup':
include/linux/printk.h:304:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/arcnet/com90xx.c:695:3: note: in expansion of macro 'pr_err'
   pr_err("Too many arguments\n");
   ^~~~~~
drivers/net/arcnet/com90xx.c:696:2: note: here
  case 3:  /* Mem address */
  ^~~~
drivers/net/arcnet/com90xx.c:697:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   shmem = ints[3];
   ~~~~~~^~~~~~~~~
drivers/net/arcnet/com90xx.c:698:2: note: here
  case 2:  /* IRQ */
  ^~~~
drivers/net/arcnet/com90xx.c:699:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   irq = ints[2];
   ~~~~^~~~~~~~~
drivers/net/arcnet/com90xx.c:700:2: note: here
  case 1:  /* IO address */
  ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
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>