mfd: intel_soc_pmic_bxtwc: Don't shadow error codes in show()/store()

kstrtox() along with regmap API can return different error codes based on
circumstances.

Don't shadow them when returning to the caller.

While at it, remove rather confusing message from addr_store().

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/20220628221747.33956-1-andriy.shevchenko@linux.intel.com