usb: gadget: uvc: Delay the status stage when setting alternate setting 1

This patch adds the support in UVC webcam gadget design for providing
USB_GADGET_DELAYED_STATUS in response to a set_interface(alt setting 1)
command issue by the Host.

The current UVC webcam gadget design generates a STREAMON event
corresponding to a set_interface(alt setting 1) command from the Host.
This STREAMON event will eventually be routed to a real V4L2 device.

To start video streaming, it may be required to perform some register
writes to a camera sensor device over slow external busses like I2C or
SPI. So, it makes sense to ensure that we delay the STATUS stage of the
set_interface (alt setting 1) command.

Otherwise, a lot of ISOC IN tokens sent by the Host will be replied to
by zero-length packets by the webcam device. On certain Hosts this may
even lead to ISOC URBs been cancelled from the Host side.

So, as soon as we finish doing all the "streaming" related stuff on the
real V4L2 device, we call a STREAMON ioctl on the UVC side and from here
we call the 'usb_composite_setup_continue' function to complete the
status stage of the set_interface(alt setting 1) command.

Further, we need to ensure that we queue no video buffers on the UVC
webcam gadget, until we de-queue a video buffer from the V4L2 device.
So, the application should call the STREAMON on UVC side only when it
has dequeued sufficient buffers from the V4L2 side and queued them to
the UVC gadget.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@st.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>