watchdog: 6300esb: add exit function

When the Intel 6300ESB watchdog is hot unplug. The timer allocated
in realize isn't freed thus leaking memory leak. This patch avoid
this through adding the exit function.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-Id: <583cde9c.3223ed0a.7f0c2.886e@mx.google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
