irq: versatile: delete dangling variable

This one was just creating noise like this:

drivers/irqchip/irq-versatile-fpga.c: In function 'fpga_irq_of_init':
drivers/irqchip/irq-versatile-fpga.c:180:24: warning: unused variable 'f'

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>