OMAP: hwmod: Rename dma_ch to dma_req

The dma request line attribute was named dma channel, which leads
to confusion with the real dma channel definition.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>