platform_get_resource() may return NULL, add proper check to
avoid potential NULL dereferencing.

Fixes: 250e2110 ("thermal: broadcom: Add Stingray thermal driver")
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Link: https://lore.kernel.org/r/20220425092929.90412-1-zhengyongjun3@huawei.comSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>