thermal: rockchip: fix a impossible condition caused by the warning

As the Dan report the smatch check the thermal driver warning:
drivers/thermal/rockchip_thermal.c:551 rockchip_configure_from_dt()
warn: impossible condition '(thermal->tshut_temp > ((~0 >> 1))) =>
(s32min-s32max > s32max)'

Although The shut_temp read from DT is u32,the temperature is currently
represented as int not long in the thermal driver.
Let's change to make shut_temp instead of the thermal->tshut_temp for
the condition.

Fixes: commit 437df217
("thermal: rockchip: consistently use int for temperatures")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>