watchdog: w83877f_wdt: Mark expected switch fall-through

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

This patch fixes the following warning:

drivers/watchdog/w83877f_wdt.c: In function ‘fop_ioctl’:
drivers/watchdog/w83877f_wdt.c:294:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   wdt_keepalive();
   ^~~~~~~~~~~~~~~
drivers/watchdog/w83877f_wdt.c:297:2: note: here
  case WDIOC_GETTIMEOUT:
  ^~~~

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

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

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

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>