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.