ALSA: seq: Degrade the error message for too many opens

ALSA OSS sequencer spews a kernel error message ("ALSA: seq_oss: too
many applications") when user-space tries to open more than the
limit.  This means that it can easily fill the log buffer.

Since it's merely a normal error, it's safe to suppress it via
pr_debug() instead.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>