usb: f_qdss: Fix race between qdss_alloc_req & qdss_close

While performing very fast etr enable/disable from coresight
there is a chance that when alloc_req is trying to allocate
the request qdss_close got called due to etr disable
invoking it, and before alloc_req could complete it's
execution driver is trying to free the request. This leads
to poison error.

Fix this by securing alloc_req under spinlock to avoid this
scenario. qdss_close is already under spinlock protection.

Change-Id: Ia2c629911c9044536a1eca5d72b6f6b3a91ef806
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>