DMA: ste_dma40: use a power of 2 check

dst_addr_width and src_addr_width should be a power of 2. Currently the
driver checks, that they both lie between 1 and 8 and that they are equal
to 1 or even. This however leaves an invalid value of 6 uncaught. Use an
explicit power of 2 check instead.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
[typo fix on changelog]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>