wl1271: Fix overflow in wl1271_boot_upload_nvs

Due to miscalculation of nvs_len, excessive data was sent to the
firmware.
Fix this by first setting nvs_ptr to point to the first NVS table,
and computing the total size of all NVS tables accordingly.

Signed-off-by: Ido Yariv <ido@wizery.com>
Tested-By: Tuomas Katila <ext-tuomas.2.katila@nokia.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>