In current implementation, AMDTP domain structure and AMDTP stream
structure has one way of reference from the former to the latter. For
future extension, bidirectional reference is needed.

This commit adds a member into stream structure to refer to domain
structure to which the stream belongs.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20200508043635.349339-4-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>