ALSA: xen-front: fix a loop timeout

We want the loop to exit when "to" is set to zero, but in the current
code it's set to -1.  Also I tweaked the indenting so it doesn't look
like we're passing "--to" to xenbus_read_unsigned().

Fixes: cc3196ae ("ALSA: xen-front: Introduce Xen para-virtualized sound frontend driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>