genirq/generic_chip: Get rid of code duplication

irq_map_generic_chip() contains about the same code as
irq_get_domain_generic_chip() except for the return values.

Split out the irq_get_domain_generic_chip() implementation so it can be
reused.

[ tglx: Removed the extra churn in irq_get_domain_generic_chip() callers
  	and massaged changelog ]

Signed-off-by: Sebastian Frias <sf84@laposte.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mason <slash.tmp@free.fr>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/579F5C69.8070006@laposte.net


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