wlcore: Refactor probe

Move most of the device-specific probe functionality into setup(), a new
op. By doing this, wlcore_probe will be the first to request a firmware
from userspace, making it easier to load the NVS file asynchronously.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>