gpio: omap: irq_shutdown: remove unnecessary call of gpiochip_unlock_as_irq

GPIOLib core implemnts irqchip->irq_request/release_resources callbacks
internally and these callbacks already contain clalls of
gpiochip_lock/unlock_as_irq().

Hence, remove unnecessary call of gpiochip_unlock_as_irq() from
omap_gpio_irq_shutdown().

Signed-off-by: Grygorii Strashko <grygorii.strashko@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>