hwmon: (fschmd) Fix a memleak on multiple opens of /dev/watchdog

When /dev/watchdog gets opened a second time we return -EBUSY, but
we already have got a kref then, so we end up leaking our data struct.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org