net: lan743x: fix duplex configuration in mac_link_up

net: lan743x: fix duplex configuration in mac_link_up

The driver does not explicitly configure the MAC duplex mode when
bringing the link up. As a result, the MAC may retain a stale duplex
setting from a previous link state, leading to duplex mismatches with
the link partner and degraded network performance.

Update lan743x_phylink_mac_link_up() to set or clear the MAC_CR_DPX_
bit according to the negotiated duplex mode.

This ensures the MAC configuration is consistent with the phylink
resolved state.

Fixes: a5f199a8d8a03 ("net: lan743x: Migrate phylib to phylink")
Signed-off-by: Thangaraj Samynathan <thangaraj.s@microchip.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20260323065345.144915-1-thangaraj.s@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>