Bluetooth: Use hci_conn_num() instead of direct connection hash access

When changing the alternate setting for the ISOC endpoints, use the
hci_conn_num() helper function to count currently established SCO
and eSCO connections and store the the value. This avoids direct
access to the connection hash.

In addition use the stored value instead accessing the connection
hash over and over again.

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