thermal: rockhip: fix setting thermal shutdown polarity

When requested thermal shutdown signal polarity is low we need to make
sure that the bit representing high level of signal is reset, and not
set all other bits in that register.

Also rename TSADCV2_INT_PD_CLEAR to TSADCV2_INT_PD_CLEAR_MASK to better
reflect its nature.

Acked-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>