pinctrl: xway: Use devm_kasprintf() instead of fixed buffer formatting

Improve readability and maintainability by replacing a hardcoded string
allocation and formatting by the use of the devm_kasprintf() helper.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190731132917.17607-4-geert+renesas@glider.be


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