pinctrl: sx150x: handle failure case of devm_kstrdup

devm_kstrdup() may return NULL if internal allocation failed.
Thus using  label, name  is unsafe without checking. Therefor
in the unlikely case of allocation failure, sx150x_probe() simply
returns -ENOMEM.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 9e80f906 ("pinctrl: Add SX150X GPIO Extender Pinctrl Driver")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>