ARM: OMAP: Proper handling of DMA4_IRQSTATUS_L0

The register bits are reset by writing one. Remove the unneeded reads and
fix writes to not clear too many bits.

Signed-off-by: Timo Teras <timo.teras@solidboot.com>
Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>