wlcore/wl12xx/wl18xx: make NVS file optional for wl18xx

Don't spew errors when we can't find the NVS file in wlcore. Instead
fail the wl12xx boot HW op if the NVS isn't found.

Take this opportunity to remove some dead code from register_hw()
which looks for the NVS again needlessly.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>