Confine CC2538 WDT on/off conf inside the driver

Instead of requiring all calls to `watchdog_start` to be
wrapped inside `#if WATCHDOG_CONF_ENABLE` guards, we control
things from within the WDT driver itself.

This commit also includes some minor documentation and
indentation cleanups
Merge pull request #682 from g-oikonomou/watchdog-cc2538

Confine CC2538 WDT on/off conf inside the driver