ALSA: pcm: 'BUG:' message unnecessarily triggers kerneloops

BugLink: http://bugs.launchpad.net/bugs/1305480

The kerneloops-daemon scans dmesg for common crash signatures, among
which is 'BUG:'. The message emitted by the PCM library is really a
warning, so the most expedient thing to do seems to be to change the
string.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>