Bluetooth: Use hci_conn_num() for checking number of LE connections

When checking for the current number of LE connections, use
hci_conn_num() function instead of a full blown lookup within
the connection hash or direct access of the counters.

In the case of re-enabling advertising, it is more useful to
check for any connection attempt or existing connection.

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