wl1271: set wl->vif only if add_interface succeeded.

set wl->vif to the newly created interface only after the firmware booted
successfully. on the way - make the function flow more clear.

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