net: dsa: tag_brcm: legacy: reorganize functions

Move brcm_leg_tag_rcv() definition to top.
This function is going to be shared between two different tags.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Link: https://patch.msgid.link/20250614080000.1884236-2-noltari@gmail.com


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