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.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1650508019-22554-1-git-send-email-baihaowen@meizu.com


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