ALSA: xen-front: Do not use stream buffer size before it is set

This fixes the regression introduced while moving to Xen shared
buffer implementation.

Fixes: 58f9d806 ("ALSA: xen-front: Use Xen common shared buffer implementation")
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: <stable@vger.kernel.org> # v5.0+
Signed-off-by: Takashi Iwai <tiwai@suse.de>