sound: oss: waveartist: simplify waveartist_sleep()

waveartist_sleep() uses loop with schedule_timeout() to unconditionally
wait for msec. Use schedule_timeout_uninteruptible() instead.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>