pinctrl: don't print unavailable function groups

There is no reason to try to print groups associated to a function if
get_function_groups returns an error. Moreover, it can lead to a NULL
pointer dereference error.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>