ALSA: firewire-lib: Add 'direction' member to 'amdtp_stream' structure

This patch adds 'direction' member to amdtp_stream structure to indicate its
direction. This patch also adds 'direction' argument to amdtp_stream_init()
function to determine its direction.

The amdtp_stream_init() function is exported and used by firewire-speakers and
dice so this patch also affects them.

This patch just add them. Actual implementation will be done by followed
patches.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>