usb: gadget: uvc: Add fix for UVC compliance test suite's assertion 6.1.25 failure

As per the UVC compliance test suite's assertion 6.1.25, the `iFunction`
field of the Interface Association Descriptor (IAD) should the match the
`iInterface` field of the standard Video Control (VC) Interface Descriptor for
this Video Interface Collection (VIC).

This mandatory case is captured in section 3.11 of the
USB Video Class Compliance specification revision 1.1

This patch fixes this test assertion's failure and has been tested on
Linux FC16, WinXP, WIN7 and WIN8 High speed and Super Speed hosts
for successful enumeration.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@st.com>
[Merged the association and control string descriptors]
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>