usb: dwc3: gadget: Let GSI ep ops allocate TRBs for GSI eps

The TRBs for GSI endpoints are allocated by the GSI ep ops.
The TRB pool allocated in the alloc_trb_pool is never used
and never freed leading to a dangling mapping in the IOMMU.
Fix this by freeing up the allocated TRB pool as part of
endpoint initialization.

Change-Id: Ia0f7b8725f5206c3c52db8017770b855361b0751
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>