wl18xx: don't upload NVS to FW

In this chip family the NVS file should not be uploaded to FW. In fact,
we encounter strange bugs (sdio errors) when trying to upload it with
certain parameters.

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