USB: gadget: diag: Use spinlocks for protection when freeing requests

There is possibility that when we do list_del in free_reqs, the list
is modified simultanuously by others. This will result in the request
not being found. Protect access using spinlocks.

Change-Id: I20c3aeabaefe8786b9b453c7b08b5c9c18057184
CRs-Fixed: 518902
Signed-off-by: Maria Yu <aiquny@codeaurora.org>