hw/dma/omap_dma: Add (uint32_t) casts when shifting uint16_t by 16

hw/dma/omap_dma: Add (uint32_t) casts when shifting uint16_t by 16

Add missing (uint32_t) casts in cases where we're trying to
put a uint16_t value into the top half of a 32-bit field.
These were already present in some but not all places.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>