Bluetooth: Track number of LE slave connections

Most (probably all) controllers can only deal with a single slave LE
connection at a time. This patch adds a counter for such connections so
that the number can be quickly looked up without iterating the
connections list.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>