Bluetooth: Fix hci_connect error return values

The hci_connect function should either return a valid hci_conn pointer
or a ERR_PTR() but never NULL. This patch fixes the two places where
hci_conn_add failures would have caused a NULL return. The only reason
for failure with hci_conn_add is memory allocation so ENOMEM seems to be
a good choice here.

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