omap: DMA: clear interrupt status correctly

When clearing the DMA channel, clear all status bits.

When handling a DMA interrupt, clear only the interrupt
status bits that have been read and are passed to the
channel's interrupt handler, not every status bit.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: G, Manjunath Kondaiah <manjugk@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>