wl12xx: start reworking the init sequence

Split the init sequence into common commands (non role-specific)
and role-specific commands.

We still need to call the common commands only on add_interface()
(rather than on start()) as the fw must get the mac address
when uploading the nvs.

Future patches will refactor the init sequence further more.

Signed-off-by: Eliad Peller <eliad@wizery.com>
[fixed a couple of sparse warnings]
Signed-off-by: Luciano Coelho <coelho@ti.com>