net: fec: enable the Jumbo frame support for i.MX8QM

net: fec: enable the Jumbo frame support for i.MX8QM

Certain i.MX SoCs, such as i.MX8QM and i.MX8QXP, feature enhanced
FEC hardware that supports Ethernet Jumbo frames with packet sizes
up to 16K bytes.

When Jumbo frames are supported, the TX FIFO may not be large enough
to hold an entire frame. To handle this, the FIFO is configured to
operate in cut-through mode when the frame size exceeds
(PKT_MAXBUF_SIZE - ETH_HLEN - ETH_FCS_LEN), which allows transmission
to begin once the FIFO reaches a certain threshold.

Reviewed-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Link: https://patch.msgid.link/20250910185211.721341-7-shenwei.wang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>