[ARM] pxa/dma: optimize irq handler loop

Reduce loop for dma irq handler callbacks to the minimum
required.

Since V1: included suggestion from Nicolas Pitre to improve
even further the loop.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>