usb: dwc3: gadget: combine unaligned and zero flags

Both flags are used for the same purpose in dwc3: appending an extra
TRB at the end to deal with controller requirements. By combining both
flags into one, we make it clear that the situation is the same and
that they should be treated equally.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>