ARM: OMAP: dma: Clear status registers on enable/disable irq

Use omap_disable_channel_irq() function instead of directly accessing CICR
register in various functions.

The omap_disable_chanel_irq() function now clears pending interrupts
and disables interrupt on channel.

Functions omap2_enable_irq_lch()/omap2_disable_irq_lch() clear interrupt
status register.

Signed-off-by: Oleg Matcovschi <oleg.matcovschi@ti.com>
Tested-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
[tony@atomide.com: updated comments to clarify CICR access]
Signed-off-by: Tony Lindgren <tony@atomide.com>