hwmon: (fschmd) Fix check on unsigned in watchdog_write()

If unsigned the watchdog_trigger() return value will not be
checked correctly.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>