mfd: intel_soc_pmic_bxtwc: Extend use of temporary variable for struct device

Extend use of temporary variable for struct device to make code neater.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220628221747.33956-5-andriy.shevchenko@linux.intel.com