rapidio: fix hang on RapidIO doorbell queue full condition

In fsl_rio_dbell_handler() the code currently simply acknowledges the QFI
queue full interrupt, but does nothing to resolve the queue full
condition.  Instead, it jumps to the end of the isr.  When a queue full
condition occurs, the isr is then re-entered immediately and continually,
forever.

The fix is to just fall through and read out current doorbell entries.

Signed-off-by: Thomas Taranowski <tom@baringforge.com>
Cc: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Li Yang <leoli@freescale.com>
Cc: Thomas Moll <thomas.moll@sysgo.com>
Cc: Micha Nelissen <micha@neli.hopto.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>