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

This patch fixes the following warnings:

drivers/staging/vt6656/key.c: In function ‘vnt_set_keymode’:
drivers/staging/vt6656/key.c:70:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   key->hw_key_idx = entry;
   ~~~~~~~~~~~~~~~~^~~~~~~
drivers/staging/vt6656/key.c:71:2: note: here
  case VNT_KEY_ALLGROUP:
  ^~~~
drivers/staging/vt6656/key.c:73:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (onfly_latch)
      ^
drivers/staging/vt6656/key.c:75:2: note: here
  case VNT_KEY_GROUP_ADDRESS:
  ^~~~
drivers/staging/vt6656/key.c:76:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
   key_mode |= mode;
   ~~~~~~~~~^~~~~~~
drivers/staging/vt6656/key.c:77:2: note: here
  case VNT_KEY_GROUP:
  ^~~~

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>