usb: gadget: uvc: Remove non-required locking from 'uvc_queue_next_buffer' routine

This patch removes the non-required spinlock acquire/release calls on
'queue->irqlock' from 'uvc_queue_next_buffer' routine.

This routine is called from 'video->encode' function (which translates to
either 'uvc_video_encode_bulk' or 'uvc_video_encode_isoc') in 'uvc_video.c'.
As, the 'video->encode' routines are called with 'queue->irqlock' already held,
so acquiring a 'queue->irqlock' again in 'uvc_queue_next_buffer' routine causes
a spin lock recursion.

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