hwmon: (nct6775) Use superio_*() function pointers in sio_data.

Prepare for platform specific callbacks usage:
* Rearrange code for directly use struct nct6775_sio_data in superio_*()
  functions.
* Use superio function pointers in nct6775_sio_data struct instead direct
  calls.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=204807
Signed-off-by: Denis Pauk <pauk.denis@gmail.com>
Co-developed-by: Bernhard Seibold <mail@bernhard-seibold.de>
Signed-off-by: Bernhard Seibold <mail@bernhard-seibold.de>
Cc: Andy Shevchenko <andriy.shevchenko@intel.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210917220240.56553-2-pauk.denis@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>