ALSA: dummy: use setup_timer and mod_timer

Use setup_timer and mod_timer instead of structure assignments as it
is the preferred way to setup and set the timer.

Signed-off-by: Roman Kollar <rkollar@mail.muni.cz>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>