Bluetooth: Fix potential memory leak

If command is added to req then it should be freed in case if
hdev is down or HCI_ADVERTISING flag is set.

This introduces a helper in hci_request to purge the cmd_q
to make cmd_q internal to hci_request which is used to fix
the leak.

This also replace accessing of cmd_q in hci_conn with the
new helper.

Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>