usb: gadget: f_uac2: fix bFirstInterface in composite gadget

usb: gadget: f_uac2: fix bFirstInterface in composite gadget

BugLink: http://bugs.launchpad.net/bugs/1783418
[ Upstream commit 8813a59ed892305b5ac1b5b901740b1ad4b5fefa ]

If there are multiple functions associated with a configuration, then
the UAC2 interfaces may not start at zero.  Set the correct first
interface number in the association descriptor so that the audio
interfaces are enumerated correctly in this case.

Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>