ALSA: usb-audio: Fix bogus error return in snd_usb_create_stream()

The function returns -EINVAL even if it builds the stream properly.
The bogus error code sneaked in during the code refactoring, but it
wasn't noticed until now since the returned error code itself is
ignored in anyway.  Kill it here, but there is no behavior change by
this patch, obviously.

Fixes: e5779998 ('ALSA: usb-audio: refactor code')
Signed-off-by: Takashi Iwai <tiwai@suse.de>