pinctrl: single: fix potential NULL dereference

In the Linux kernel, the following vulnerability has been resolved: pinctrl: single: fix potential NULL dereference Added checking of pointer "function" in pcs_set_mux(). pinmux_generic_get_function() can return NULL and the pointer "function" was dereferenced without checking against NULL. Found by Linux Verification Center (linuxtesting.org) with SVACE.