pinctrl: check the return value of pinmux_ops::get_function_name()

In the Linux kernel, the following vulnerability has been resolved: pinctrl: check the return value of pinmux_ops::get_function_name() While the API contract in docs doesn't specify it explicitly, the generic implementation of the get_function_name() callback from struct pinmux_ops - pinmux_generic_get_function_name() - can fail and return NULL. This is already checked in pinmux_check_ops() so add a similar check in pinmux_func_name_to_selector() instead of passing the returned pointer right down to strcmp() where the NULL can get dereferenced. This is normal operation when adding new pinfunctions.