irqchip: xtensa: Select only an online cpu

The user space interface does not filter out offline cpus. It merily
verifies that the mask contains at least one online cpu. So the
selector in the irq chip implementation needs to make sure to pick
only an online cpu because otherwise:

     Offline Core 1
     Set affinity to 0xe
     Selector will pick first set bit, i.e. core 1

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: xtensa <linux-xtensa@linux-xtensa.org>