net: fec: update MAX_FL based on the current MTU

Configure the MAX_FL (Maximum Frame Length) register according to the
current MTU value, which ensures that packets exceeding the configured MTU
trigger an RX error.

Reviewed-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Link: https://patch.msgid.link/20250910185211.721341-4-shenwei.wang@nxp.com


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