ARM: OMAP: DMA: Fix incorrect channel linking

Function enable_lnk does incorrect channel link on non-omap1 builds if chain
is created manually with omap_request_dma and omap_dma_link_lch functions.

Fix this by making sure that next_linked_ch field is initialized to -1 just
in omap_request_dma.

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