thermal: rockchip: use the usleep_range instead of udelay

Documentation/timers/timers-howto.txt recommends to use
usleep_range on delays > 10usec.
The usleep_range indeed reduces CPU load, since the udelay will busy wait
for enough loop cycles to achieve the desired delay.

Fixes commit b06c52db39fd ("thermal: rockchip:
handle the power sequence for tsadc controller").

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Suggested-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>