ath6kl: use custom MAC address for newly created interfaces

Firmware and driver generate MAC addresses for the second and third interfaces.
In addition to the existing algorithm, flip bit 7 of 5th octet. Since both
firmware and driver individually generate the MAC addresses, introduce a new
firmware capability bit to keep them compatible.

Signed-off-by: Aarthi Thiruvengadam <athiruve@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>