net: pcs: xpcs: fix the wrong register that was written back

The value is read from the register TXGBE_RX_GEN_CTL3, and it should be
written back to TXGBE_RX_GEN_CTL3 when it changes some fields.

Cc: stable@vger.kernel.org
Fixes: f629acc6 ("net: pcs: xpcs: support to switch mode for Wangxun NICs")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reported-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20240924022857.865422-1-jiawenwu@trustnetic.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>