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().