ALSA: sb: Minor optimization / fix of timer usage in sb8_midi.c

ALSA: sb: Minor optimization / fix of timer usage in sb8_midi.c

Currently the SB8 MIDI code sets up the timer object at each time
before scheduling it at trigger callback, but basically this is
superfluous once after set up.  Also, the code misses the
del_timer_sync() call that may leave a race condition for
use-after-free.

This patch addresses these issues, moving timer_setup() to
snd_sb8dsp_midi(), and adding the del_timer_sync() call at
snd_sb8dsp_midi_output_trigger() to make sure.

Signed-off-by: Takashi Iwai <tiwai@suse.de>