[PATCH v2]: watchdog: sunxi: Allow watchdog to remain enabled after probe

I (tried) to fix commit format (still learning, please comment is something
is missing).

I made the suggested code fixes and added a sunxi_wdt_read_timeout function;
it was a bit more touchy, as the HW timeout could be set to less than a second
(really?), so I had to manage max_hw_heartbeat_ms too.