irqchip: sirf: set IRQ_LEVEL status_flags

SiRF internal interrupts are using level trigger. we need to tell the irq
core this information. otherwise, we might get some problems as below
1. disable_irq(n)
here irq core will mark the disabled flag but still keep the irq enabled
due to involved lazy-disable
2. doing someting after disable_irq(n)
in step 2, if one interrupt n comes, irq core will mark it as pending and
mask the HW interrupt really. we name the coming interrupt as "X".
3. enable_irq(n)
this will unmask the interrupt, so the level-trigger HW interrupt will come
again, irq_handler will enter as "E1". after that, irq core will also check
whether irq n is pending, if yes, and pending interrupt is not level-trigger,
irq core will execute the pending irq_handler.
so if we don't set the IRQ_LEVEL flag here, irq core will execute pending
X again as "E2", but actually the pending interrupt has been handled by "E1".
that makes a level-trigger HW interrupt is executed twice.

here we fix the issue to avoid redundant interrupt overload.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Huayi Li <Huayi.Li@csr.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

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