gpio: mvebu: Slightly optimize mvebu_gpio_irq_handler()

gpio: mvebu: Slightly optimize mvebu_gpio_irq_handler()

In the main loop of mvebu_gpio_irq_handler() some calls to
irq_find_mapping() can be saved.

There is no point to find an irq number before checking if this something
has to be done.
By testing first, some calls can be saved.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/7190f5def0489ed3f40435449c86cd7c710e6dd4.1760862679.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>