Bluetooth: hci_ath: Add ath_vendor_cmd helper

Introduce ath_vendor_cmd function which can be used to
configure 'tags' and patch the firmware.

ATH vendor command has the following format:
| OPCODE (u8) | INDEX (LE16) | DLEN (U8) | DATA (U8 * DLEN) |

BD address configuration tag is at index 0x0001.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>