usb: gadget: u_audio: Move dynamic srate from params to rtd

Parameters uac_params.p_srate/c_srate are dynamic now and are not part
of parametric configuration anymore. Move them to the
runtime struct uac_rtd_params for each stream.

Suggested-by: John Keeping <john@metanate.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-4-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>