pinctrl: single: Fix a couple error codes

We should return -ENOMEM instead of success if pcs_add_function() fails.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>