mfd: intel_soc_pmic_bxtwc: Use bits.h macros for all masks

Currently we are using BIT(), but GENMASK(). Make use of the latter one
as well (far less error-prone, far more concise).

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/20220628221747.33956-8-andriy.shevchenko@linux.intel.com