thermal/drivers/intel/intel_soc_dts_iosf: Switch to use find_first_zero_bit()

Switch to use find_first_zero_bit() instead of open-coded variant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210618153451.89246-1-andriy.shevchenko@linux.intel.com