ARM: omap: remove almost-const variables

dma_stride and dma_common_ch_start are only ever initialised to one
known value at initialisation, and are private to each of these files.
There's no point these being variables at all.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>