w1: w1_therm: Use max() instead of doing it manually

Fix following coccicheck warning:
drivers/w1/slaves/w1_therm.c:1452:18-19: WARNING opportunity for max()

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1646908169-8050-1-git-send-email-baihaowen@meizu.com


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>