pinctrl: mcp23s08: Refactor mcp23s08_spi_regmap_init()

There is a lot of duplication for one small helper function.
Refactor mcp23s08_spi_regmap_init() to prepare everything first
and then register regmap at the end.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200407173849.43628-5-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>