sparc: Use the new genirq functionality

Make use of the new features in genirq:

1) Set the chip flag IRCHIP_EOI_IF_HANDLED, which ensures in the
   core code that irq_eoi() is only called when the interrupt was
   handled. That removes the extra status check in the callback.

2) Use the preflow handler, which is called from the fasteoi core code
   before the device handler. That avoids another status check and the
   open coded handler redirection.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: sparclinux@vger.kernel.org