wl12xx: 1281/1283 support - Loading FW & NVS

Take care of FW & NVS with the auto-detection between wl127x and
wl128x.

[Moved some common code outside if statements and added notes about
NVS structure assumptions; Fixed a bug when checking the nvs size: if
the size was incorrect, the local nvs variable was set to NULL, it
should be wl->nvs instead. -- Luca]

[Merged with potential buffer overflow fix -- Luca]

Signed-off-by: Shahar Levi <shahar_levi@ti.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>