Bluetooth: hci_conn: Reduce hci_conn_drop() calls in two functions

An hci_conn_drop() call was immediately used after a null pointer check
for an hci_conn_link() call in two function implementations.
Thus call such a function only once instead directly before the checks.

This issue was transformed by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>