wl12xx: refactor fw init into a new function

The fw boot and initialization currently happens inside the
add_interface() callback. This is wrong, as add_interface is
called for each new vif. However, we due to some fw limitation
(we have to know the actual mac address on boot), we can't
completely move it into the start() callback.

Until the fw will be fixed, refactor the fw init into
a new function, and call it from add_interface()

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>