pinctrl/abx500: destroy mutex if returning early due to error

Current failure path neglects to mutex_destroy() before returning
an error due to an invalid parameter or an error received from
gpiochip_add(). This patch aims to remedy that behaviour.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>