wl1271: upload only the first 468 bytes from the NVS file

We were uploading the whole NVS file, but that is wrong, because the same
file also contains the initialization values.  For the latest firmwares, we
should upload only the initial 468 bytes from the file.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>