libertas_tf: get the MAC address before registering the device

The start() callback is too late for this: NetworkManager would already
have seen the hardware, thinking 00:00:00:00:00:00 is its permanent
address.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Steve deRosier <derosier@cal-sierra.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>