net: fec: use a member variable for maximum buffer size

Refactor code to support Jumbo frame functionality by adding a member
variable in the fec_enet_private structure to store PKT_MAXBUF_SIZE.

Remove the OPT_FRAME_SIZE and define a new macro OPT_ARCH_HAS_MAX_FL to
indicate architectures that support configurable maximum frame length.
And update the MAX_FL register value to max_buf_size when
OPT_ARCH_HAS_MAX_FL is defined as 1.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Link: https://patch.msgid.link/20250910185211.721341-2-shenwei.wang@nxp.com


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