thermal/core: Precompute the delays from msecs to jiffies

The delays are stored in ms units and when the polling function is
called this delay is converted into jiffies at each call.

Instead of doing the conversion again and again, compute the jiffies
at init time and use the value directly when setting the polling.

Cc: Thara Gopinath <thara.gopinath@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Thara Gopinath <thara.gopinath@linaro.org>
Link: https://lore.kernel.org/r/20201216220337.839878-1-daniel.lezcano@linaro.org