avr32/irq: Use irq_desc_get_xxx() to avoid redundant lookup of irq_desc

Use irq_desc_get_xxx() to avoid redundant lookup of irq_desc while we
already have a pointer to corresponding irq_desc.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Link: http://lkml.kernel.org/r/20150713100606.527106283@linutronix.de


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