pinctrl: mcp23s08: fix probing of mcp23s18

one_regmap_config is always null if mcp type is MCP_TYPE_S18.
Remove the null check so that the mcp23s18 will probe.

Fixes: 1781af56 ("pinctrl: mcp23s08: spi: Fix duplicate pinctrl debugfs entries")
Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>