pinctrl: at91: Fix possible out-of-boundary access

In the Linux kernel, the following vulnerability has been resolved: pinctrl: at91: Fix possible out-of-boundary access at91_gpio_probe() doesn't check that given OF alias is not available or something went wrong when trying to get it. This might have consequences when accessing gpio_chips array with that value as an index. Note, that BUG() can be compiled out and hence won't actually perform the required checks.