pinctrl: single: fix potential NULL dereference in pcs_get_function()

In the Linux kernel, the following vulnerability has been resolved: pinctrl: single: fix potential NULL dereference in pcs_get_function() pinmux_generic_get_function() can return NULL and the pointer 'function' was dereferenced without checking against NULL. Add checking of pointer 'function' in pcs_get_function(). Found by code review.