mfd: intel_soc_pmic_bxtwc: Use sysfs_emit() instead of sprintf()

sysfs_emit() is preferred over sprintf() when formatting the value to be
returned to user space in show() functions, because it knows about sysfs
buffer specifics and has sanity checks.

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/20220628221747.33956-9-andriy.shevchenko@linux.intel.com