qemu-char: Fix ringbuf option size

Any attempt to use it trips an "opt->desc->type == QEMU_OPT_NUMBER"
assertion.  Broken in commit 1da48c65.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
(cherry picked from commit 0f953051)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>