i.MX31/Gigabeat S: The nested IRQ code was just totally wrong and not actually workin...

i.MX31/Gigabeat S: The nested IRQ code was just totally wrong and not actually working anyway (which is why it wasn't crashing). AVIC doesn't seem truthful about priority of current ISR either :\. Sometimes there were channel swaps during really active DVFS due to FIFO underflow from a needed long delay in the ISR (100us, and IIS TX only has 45us reserve) and DMARQ *must* be serviced low-latency. Get it doing as was intended.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@25822 a1c6a512-1295-4272-9138-f99709370657