watchdog: change close handling

The internal watchdog should only works if the file is properly closed. So far,
the internal watchdog is startet by closing /dev/watchdog. With this commit
the letter 'V' must be written or stopping user watchdog must be done by an
ioctl function call.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>