usb: gadget: u_audio: remove struct uac_req

'struct uac_req' purpose is to link 'struct usb_request' to the
corresponding 'struct uac_rtd_params'. However member req is never
used. Using the context of the usb request, we can keep track of the
corresponding 'struct uac_rtd_params' just as well, without allocating
extra memory.

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20210118084642.322510-4-jbrunet@baylibre.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>