arm: plat-omap: dma: make omap_dma_in_1510_mode() static

Eliminates the following sparse warning:

	arch/arm/plat-omap/dma.c:137:5: warning: symbol 'omap_dma_in_1510_mode' was not declared. Should it be static?

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>