[ALSA] gusextreme: set codec_flag

The gusextreme driver neglects to set the gus->codec_flag meaning
snd_gf1_pcm_new() allocates a second 'PCM Playback Volume' control,
which makes the driver fail to load.

Signed-off-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>