pinctrl: SPEAr: Update error check for unsigned variables

Checking '< 0' for unsigned variables always returns false. For error
codes, use IS_ERR_VALUE() instead.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>