USB: dwc3: msm: Check endpoint status and request NULL or not

Check for endpoint disable first and then whether request
being queued is NULL or not before queuing request to USB HW.
Also enhance spinlock protection to avoid any race with endpoint
disable and request being freed.

Change-Id: Ic79a0de53d1dc8881877428d9f6fe0c58959b182
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>