dmaengine: ti-dma-crossbar: make omap_dmaxbar_init static

The omap_dmaxbar_init() function is not exported or declared outside
the driver, so make it static to fix the following sparse warning:

drivers/dma/ti-dma-crossbar.c:455:5: warning: symbol 'omap_dmaxbar_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>