dmaengine: coh901318: statify symbols

Sparse complains:

drivers/dma/coh901318.c:269:30: warning: symbol 'chan_config' was not declared. Should it be static?
drivers/dma/coh901318.c:2806:12: warning: symbol 'coh901318_init' was not declared. Should it be static?
drivers/dma/coh901318.c:2812:13: warning: symbol 'coh901318_exit' was not declared. Should it be static?

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>