USB: gadget: Handle the return value if ep_queue fails for ZLP

Currently diag and qdss drivers are not checking the return value
when it queues zlp in interrupt context for TX endpoint. This could
result in memory leak if ep_queue fails for zlp. Hence check the
return value of usb_ep_queue() and handle the error case.

CRs-Fixed: 550211
Change-Id: I9f0be89ffbadaa57c9e4dd264651ea44cdc4ec27
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>