omap: dma: Add read-back to DMA interrupt handler to avoid spuriousinterrupts

Flush the writes to IRQSTATUS_L0 register in the DMA interrupt handler by reading the register
directly after write. This prevents the spurious DMA interrupts noted when using VDD_OPP 1

Signed-off-by: Mathias Nyman <mathias.nyman@nokia.com>
Acked-by: Santosh Shilimkar <Santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>