wl12xx: read chip ID and HW PG version during probe

In order to read the MAC addresses from the fuse ROM, we need to know
the chip ID and the HW PG version.  We need to know the MAC address
during probe, because that's when we register our HW with mac80211.

To prepare for that, this patch reads the chip ID and HW PG version
during probe instead of doing it at boot time.  We power the chip on
briefly in order to do that.

Signed-off-by: Luciano Coelho <coelho@ti.com>