USB: gadget: f_uac2: Fix broken prm to uac2 mapping

prm_to_uac2() is broken because it tests against pointer it itself
mapped onto, which will never be different.
Fix the mapping by adding pointer to parent chip in each rtd param
and removing the prm_to_uac2().

Reported-by: Julien Rouviere <jrouviere@qualistream.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>