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>