dmaengine: ste_dma40_ll: make d40_width_to_bits static

Fix warning due to d40_width_to_bits() not being used outside
this file. Fixes:

drivers/dma/ste_dma40_ll.c:13:4: warning: symbol 'd40_width_to_bits' 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>

drivers/dma/ste_dma40_ll.c

@@ -10,7 +10,7 @@
 
 #include "ste_dma40_ll.h"
 
-u8 d40_width_to_bits(enum dma_slave_buswidth width)
+static u8 d40_width_to_bits(enum dma_slave_buswidth width)
 {
 	if (width == DMA_SLAVE_BUSWIDTH_1_BYTE)
 		return STEDMA40_ESIZE_8_BIT;