brcmfmac: insert default boardrev in nvram data if missing

Some nvram files/stores come without the boardrev information,
but firmware requires this to be set. When not found in nvram then
add a default boardrev string to the nvram data.

Reported-by: Rafal Milecki <zajec5@gmail.com>
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <franky.lin@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Signed-off-by: Hante Meuleman <hante.meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>