[BLUETOOTH]: Add conn add/del workqueues to avoid connection fail.

The bluetooth hci_conn sysfs add/del executed in the default
workqueue.  If the del_conn is executed after the new add_conn with
same target, add_conn will failed with warning of "same kobject name".

Here add btaddconn & btdelconn workqueues, flush the btdelconn
workqueue in the add_conn function to avoid the issue.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>