irqchip: orion: Reverse irq handling priority

Non-DT irq handlers were working through irq causes from most-significant
to least-significant bit, while DT irqchip driver does it the other way
round. This revealed some more HW issues on Kirkwood peripheral IP, where
spurious sdio irqs can happen although irqs are masked.

Also, the generated binaries show that original non-DT order compared
to DT order save two instructions for each bit count check:

irqchip DT order with ffs():
  60:   e3a06001        mov     r6, #1
  64:   e2643000        rsb     r3, r4, #0
  68:   e0033004        and     r3, r3, r4
  6c:   e16f3f13        clz     r3, r3
  70:   e263301f        rsb     r3, r3, #31
  74:   e1c44316        bic     r4, r4, r6, lsl r3
  78:   e5971004        ldr     r1, [r7, #4]

Original non-DT order with fls():
  60:   e3a07001        mov     r7, #1
  64:   e16f3f14        clz     r3, r4
  68:   e263301f        rsb     r3, r3, #31
  6c:   e1c44317        bic     r4, r4, r7, lsl r3
  70:   e5951004        ldr     r1, [r5, #4]

Therefore, reverse irq bit handling back to original order by replacing
ffs() with fls().

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Link: https://lkml.kernel.org/r/1398719528-23607-1-git-send-email-sebastian.hesselbarth@gmail.com
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

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