pinctrl: mcp23s08: Consolidate SPI and I²C code

There are three parts of each, SPI and I²C, driver spread over the code under
ifdeffery. Consolidate them so it will be only one for SPI and one for I²C.

The code has been cosmetically changed to avoid intrusive change.
Clean up is considered in the nearest future.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200407173849.43628-3-andriy.shevchenko@linux.intel.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>