usb: dwc3: gadget: Fix desc NULL pointer in dwc3_gadget_ep_queue()

dep->endpoint.desc is checked at the beginning of
dwc3_gadget_ep_queue(), but after that it may be set to NULL
by another thread and then accessed again in dwc3_gadget_ep_queue().
This will lead to kernel oops.

Expand spinlock protection area to aviod race condition.

Signed-off-by: Zhuang Jin Can <jin.can.zhuang@intel.com>
Signed-off-by: Jiebing Li <jiebing.li@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>