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

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: [email protected]
Fixes: f629acc6f210 ("net: pcs: xpcs: support to switch mode for Wangxun NICs")
Signed-off-by: Jiawen Wu <[email protected]>
Reported-by: Russell King (Oracle) <[email protected]>
Reviewed-by: Russell King (Oracle) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>