usb: dwc3: Allocate TRB pool upon enabling endpoint

Currently gadget driver is by default allocating dma TRB
pool per endpoint for 30 endpoints upon driver probe.
Optimize this by allocating dma memory only for enabled
endpoints.

Change-Id: I378a42d0b84fa43f7c20cd025cd9361c467cb41b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>