mfd: intel_soc_pmic_bxtwc: Drop unneeded casting

The casting from size_t to ssize_t is not needed in addr_store(),
drop it.

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/20220628221747.33956-10-andriy.shevchenko@linux.intel.com