gpio: sifive: Look up IRQs only once during probe

of_irq_count(), or eqivalently platform_irq_count(), simply looks up
successively-numbered IRQs until that fails. Since this driver needs to
look up each IRQ anyway to get its virq number, use that existing loop
to count the IRQs at the same time.

The check against SIFIVE_GPIO_MAX functioned as a bounds check for
chip->irq_number. That is now handled by the loop condition.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>