[MIPS] Sibyte: Improve interrupt latency again for sb1250/bcm1480

this patch restores the behaviour of the old (assembly-written)
interrupt handler, the handler is left as soon as a single interrupt
cause is handled.

Signed-off-by: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>