Bluetooth: Move HCI_RUNNING check into hci_send_frame

In all callbacks for hdev->send the status of HCI_RUNNING is checked. So
instead of repeating that code in every driver, move the check into the
hci_send_frame function before calling hdev->send.

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