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

Documentation/ABI/testing/configfs-usb-gadget-uac2

@@ -30,4 +30,6 @@ Description:
 					(in 1/256 dB)
 		p_volume_res		playback volume control resolution
 					(in 1/256 dB)
+		req_number	the number of pre-allocated requests for both capture
+					and playback
 		=====================	=======================================