MIPS: lantiq: xway: mark dma_init() as static

Fix the following missing-prototypes build warning:

arch/mips/lantiq/xway/dma.c:293:1: error: no previous prototype for 'dma_init' [-Werror=missing-prototypes]
  293 | dma_init(void)
      | ^~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/lantiq/xway/dma.c

@@ -289,7 +289,7 @@ static struct platform_driver dma_driver = {
 	},
 };
 
-int __init
+static int __init
 dma_init(void)
 {
 	return platform_driver_register(&dma_driver);