thermal: sun8i: Use bitmap API instead of open code

The bitmap_* API is the standard way to access data in the bitfield.
So convert irq_ack to return an unsigned long, and make things to use
bitmap API.

Signed-off-by: Yangtao Li <frank@allwinnertech.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201109114624.23035-1-frank@allwinnertech.com