amdsbwd: fix handling of timeout values beyond the supported range

The driver now fully observes watchdog(9) protocol.
Previously a too large timeout was silently clamped while the correct
behavior is to disable the watchdog and leave the error as is
(i.e. to not report success).

Also, previously a too small value caused the timer to stop while the
correct behavior is to use the minimal supported value.

MFC after: 2 weeks