greybus: gpio: handle set_irq_flags api change

The ARM-specific set_irq_flags helper has been removed in 4.3. Instead
of doing conditional compilation on the kernel version to avoid build
breakages, simply use the genirq interface directly.

Suggested-by: Rob Herring <rob.herring@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>