usb: dwc3: gadget: stop touching HWO TRBs

Say we have three requests prepared to the HW (reqA,
reqB, and reqC). All of them are composed of
SG-lists with several entries and they all requests
interrupt only on last TRBs of the SG-list.

When we get interrupt for reqA, it could be that
reqB is already half-way transferred and some of its
TRBs will have HWO already cleared.

It's okay to free up TRBs without HWO bit set, but
we need to guarantee we don't giveback a request
that's half-way transferred as that will confuse
gadget drivers.

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