USB: gadget: midi: Fix use-after-free after midi_unbind

If midi composition is unbound while streaming is going
on then there is a possibility of in/out_trigger pcm
routines accessing midi structure after it is already
freed on function unbind.

CRs-fixed: 2020461
Change-Id: Ifb3141f3a26f073f6df7c4ae7e821c828d723a4f
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>