ARM: OMAP: DMA: Don't mark channel active in omap_enable_channel_irq

Channel should be marked active only when DMA is really started. Otherwise
just omap_request_dma, omap_dma_link_lch and omap_dma_unlink_lch will cause
incorrect dump_stack().

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>