If get_bpos() fails, it is likely that the corresponding error code should
be returned.

Fixes: a6970bb1 ("ALSA: gus: Convert to the new PCM ops")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/d9ca841edad697154afa97c73a5d7a14919330d9.1727984008.git.christophe.jaillet@wanadoo.frSigned-off-by: Takashi Iwai <tiwai@suse.de>