rk_tsadc: use tsadc_temp_to_raw for shutdown_temp

Use tsadc_temp_to_raw instead of tsadc_raw_to_temp, otherwise rk3566/68
get stuck in a kernel reboot cycle.

https://forums.freebsd.org/threads/running-freebsd-on-radxa-rock-3c-rk3566-board.89389/post-624890

Submitted by: covacat
Reviewed by: mmel
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52309