platform/x86: acer-wmi: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/platform/x86/acer-wmi.c: In function ‘set_u32’:
drivers/platform/x86/acer-wmi.c:1378:33: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (cap == ACER_CAP_WIRELESS ||
                                 ^
drivers/platform/x86/acer-wmi.c:1386:3: note: here
   case ACER_WMID:
   ^~~~
drivers/platform/x86/acer-wmi.c:1393:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
    else if (wmi_has_guid(WMID_GUID2))
            ^
drivers/platform/x86/acer-wmi.c:1395:3: note: here
   default:
   ^~~~~~~
drivers/platform/x86/acer-wmi.c: In function ‘get_u32’:
drivers/platform/x86/acer-wmi.c:1340:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (cap == ACER_CAP_MAILLED) {
      ^
drivers/platform/x86/acer-wmi.c:1344:2: note: here
  case ACER_WMID:
  ^~~~
drivers/platform/x86/acer-wmi.c: In function ‘WMID_get_u32’:
drivers/platform/x86/acer-wmi.c:1013:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (quirks->mailled == 1) {
      ^
drivers/platform/x86/acer-wmi.c:1018:2: note: here
  default:
  ^~~~~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>