usb: dwc3: gadget: prepare TRBs on update transfers too

If we're updating transfers, we can also prepare as
many TRBs as we can fit in the ring. Let's start
doing that.

This patch 'solves' a limitation of how many TRBs we
can prepare when we're getting close the end of the
ring. Instead driver to prepare only up to end of
the ring, we check if we have space to wrap around
the ring properly.

Note that this only happens when our enqueue and
dequeue pointers are equal (which is the case for
bulk endpoints after an XferComplete event).

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