thermal/drivers/sprd: Fix return value of sprd_thm_probe()

When call function devm_platform_ioremap_resource(), we should use IS_ERR()
to check the return value and return PTR_ERR() if failed.

Fixes: 554fdbaf ("thermal: sprd: Add Spreadtrum thermal driver support")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1590371941-25430-1-git-send-email-yangtiezhu@loongson.cn