arch/hexagon/kernel/dma.c: make function static

This function is only used in the same file, and the other similar
functions in this file are also static.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
[rkuo@codeaurora.org: adjusted patch to apply to latest]
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>