pinctrl: mcp23s08: Print error message when regmap init fails

It is useful for debugging to have the error message printed
when regmap initialisation fails. Add it to the driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20201009180856.4738-2-andriy.shevchenko@linux.intel.com


Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>