usb: gadget: uvc: Use GFP_ATOMIC under spin lock

Found using the following semantic patch:
<spml>
@@
@@
spin_lock_irqsave(...);
... when != spin_unlock_irqrestore(...);
* GFP_KERNEL
</spml>

Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>