drivers: watchdog: sam0: Check if timeout is valid

The upper limit of the timeout should not be 0.
tests/drivers/watchdog/wdt_basic_api checks for this and fails as the
driver currently only checks that the timout does not exceed the upper
bound.
This also makes it check the lower bound, so that the test passes.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>