net: pcs: xpcs: rearrange xpcs_link_up_1000basex()

Rearrange xpcs_link_up_1000basex() to make it more obvious what will
happen in the following commit.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>