pinctrl: spear: Make get_gpio_pingroup return NULL when no gpio_pingroup found

Currently get_gpio_pingroup() may return NULL or ERR_PTR(-EINVAL) when
no gpio_pingroup found. The caller in gpio_request_endisable() only
checks if the return value is NULL. Return ERR_PTR(-EINVAL) for
get_gpio_pingroup() causes problem and seems not necessary.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>