ALSA: firewire-motu: unify common tracepoints for both direction

In ALSA firewire-motu driver, some tracepoints are supported to probe
vendor-specific data fields for SPH and status/control messages in
payload of isochronous packet. At present, the events of tracepoints
are unique each of direction, however the pair of events has the
same structure and print format. It's possible to unify the pair.

This commit unifies the pair. From userspace, direction is specified
by filtering for src/dst fields in the structure.

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