net: airoha: Move min/max packet len configuration in airoha_dev_open()

In order to align max allowed packet size to the configured mtu, move
REG_GDM_LEN_CFG configuration in airoha_dev_open routine.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250304-airoha-eth-rx-sg-v1-1-283ebc61120e@kernel.org


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