net: stmmac: dwmac4: fix PCS duplex mode decode

dwmac4 was decoding the duplex mode from the GMAC_PHYIF_CONTROL_STATUS
register incorrectly, using GMAC_PHYIF_CTRLSTATUS_LNKMOD_MASK (value 1)
rather than GMAC_PHYIF_CTRLSTATUS_LNKMOD (bit 16). Fix this.

Fixes: 70523e63 ("drivers: net: stmmac: reworking the PCS code.")
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1sbJvd-001rGD-E3@rmk-PC.armlinux.org.uk


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