ARM: OMAP: DMA: Fix uninitialized channel flags

This has similar symptoms than 66c23551
where just omap_request_dma, omap_dma_link_lch and omap_dma_unlink_lch
can cause incorrect dump_stack(). Here it can happen if channel has been
used before and the channel flags variable holds old status.

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