libertas_tf: don't defer firmware loading until start()

In order to be able to get a MAC address before we register the device
with ieee80211 we'll need to load the firmware way earlier.

There seems to be one problem with this: the device seems to start
with radio enabled and starts sending in frames right after the firmware
load finishes. Disable the radio as soon as possible.

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