docs: ABI: added missing num_requests param to UAC2

The existing configfs-usb-gadget-uac2 ABI doc for testing was missing
the num_requests param. The patch adds the parameter to the document.

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20211225130929.205629-1-pavel.hofman@ivitera.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>