mfd: intel_soc_pmic_bxtwc: Convert to use platform_get/set_drvdata()

We have the specific helpers for platform device to set and get
its driver data. Convert driver to use them instead of open coded
variants.

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/20220628221747.33956-3-andriy.shevchenko@linux.intel.com