watchdog: w83977f_wdt: underflow in wdt_set_timeout()

"t" is controlled by the user.  If "t" is a very large integer then it
could lead to a negative "tmrval".  We cap the upper bound of "tmrval"
but, in the current code, we allow negatives.  This is a bug and it
causes a static checker warning.  Let's make "tmrval" unsigned to avoid
this problem.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>