pinctrl: at91: Fix failure path in at91_gpio_probe path

This fixes the whole error handling in probe function by capturing and
returning error values on kernel function like clk_prepare,
clk_enable, gpiochip_add etc.

CC: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
CC: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pramod Gurav <pramod.gurav@smartplayin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>