ARM: mx5: board-cpuimx51.c fixup irq_to_gpio() usage

irq_to_gpio() is being called on a GPIO so change to using
gpio_to_irq() instead.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>