staging: ti-soc-thermal: fix bitfield writing while updating thresholds

Wrong threshold cold values may be written with current implementation.
This patch fixes the threshold update function by simplifying the
bitfield manipulation sequence.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>