Split wlan_add_card() into a part that just setups kernel parameters and
into the function libertas_activate_card(), which will implizitly use
hardware functions by the started thread.

This allows us later to do something like this:

priv = libertas_add_card();
priv->hw_command_to_host = if_usb_command_to_host;
priv->hw_xxxx = if_usb_xxxx;
priv->hw_yyyy = if_usb_yyyy;
wlan_activate_card()

and of course the CF driver can set it's own functions.
Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>