pinctrl: at91: simplify probe error handling

There is no code ender the 'err' label. Just return the error code directly.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>