[ALSA] seq: resource leak fix and various code cleanups

This patch fixes:
1) a resource leak (CID: 1817)
2) various code cleanups

Signed-off-by: Eugene Teo <eugeneteo@kernel.sg>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>