dmaengine: pl08x: use GENMASK() to create bitmasks

This switches the arbitrary shifting of hex constants in
the pl080 header to use GENMASK().

Suggested-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>