The Watchdog on the CC2538 only supports 4 intervals (2ms, 16ms, 250ms & 1s).
Since the watchdog timer API specifies a `max_time`, the interval equal or
below that time is selected.
E.g. for `max_time=125ms` the 16ms interval would be selected.
This is outside the tolerance of the `tests/periph_wdt` test.
Co-authored-by: Thomas Geithner <thomas.geithner@dai-labor.de>
