ALSA: firewire-digi00x: simplify error path to begin streaming session

This commit is a part of preparation to perform allocation/release
of isochronous resources in pcm.hw_params/hw_free callbacks.

The caller of begin_session() calls finish_session() in its error path,
thus no need to call finish_session() in error path of begin_session().

This commit simplifies error path of begin_session().

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