thermal/drivers/imx8mm_thermal: Use GENMASK() when appropriate

GENMASK() is preferred to use for bitmasks.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20221014081620.1599511-1-marcus.folkesson@gmail.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>