net: fec: fec_restart(): introduce a define for FEC_ECR_SPEED

Replace "1 << 5" for configuring 1000 MBit/s with a defined constant to
improve code readability and maintainability.

Reviewed-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://patch.msgid.link/20250618-fec-cleanups-v4-6-c16f9a1af124@pengutronix.de


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