usb: dwc3: set dma_mask for dwc3_omap device

*dma_mask* is not set for devices created from dt data. So filled dma_mask
for dwc3_omap device here. And dwc3 core will copy the dma_mask from its
parent.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>