powerpc: cell: Fix the irq_desc access

The core irq_set_type() function updates the flow type when the chip
callback returns 0. It also updates irq_data, so this can be used in
irq_ack() to check for the level bit. That avoids a redundant sparse
irq lookup.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>