bt-host: add missing break statement

The switch statement in bt_host_read() is missing a break in one case.
Andrzej Zaborowski <andrew.zaborowski@intel.com> confirmed that this is
not an intentional fall-through.

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>