pinctrl: mcp23s08: check return value of devm_kasprintf()

pinctrl: mcp23s08: check return value of devm_kasprintf()

devm_kasprintf() returns a pointer to dynamically allocated memory.
Pointer could be NULL in case allocation fails. Check pointer validity.
Identified with coccinelle (kmerr.cocci script).

Fixes: 0f04a81784fe ("pinctrl: mcp23s08: Split to three parts: core, I²C, SPI")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230621100409.1608395-1-claudiu.beznea@microchip.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>