MIPS: Sibyte: Fix locking in set_irq_affinity

Locking of irq_desc is now done in irq_set_affinity; don't lock it again
in chip specific set_affinity function.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>