net: dsa: b53: prevent BRCM_HDR access on older devices

Older switches don't implement BRCM_HDR register so we should avoid
reading or writing it.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Link: https://patch.msgid.link/20250614080000.1884236-11-noltari@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>