pinctrl: aspeed: Fix potential NULL dereference in aspeed_pinmux_set_mux()

In the Linux kernel, the following vulnerability has been resolved: pinctrl: aspeed: Fix potential NULL dereference in aspeed_pinmux_set_mux() pdesc could be null but still dereference pdesc->name and it will lead to a null pointer access. So we move a null check before dereference.