usb: gadget: f_midi: Use refcount when freeing f_midi_opts

Currently, the midi function is not freed until it is
both removed from the config and released by the user.
Since the user could take a long time to release the
card, it's possible that the function could be unlinked
and thus f_midi_opts would be null when freeing f_midi.

Thus, refcount f_midi_opts and only free it when it is
unlinked and all f_midis have been freed.

Signed-off-by: Jerry Zhang <zhangjerry@google.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>