net: pcs: xpcs: combine xpcs_link_up_{1000basex,sgmii}()

xpcs_link_up_sgmii() and xpcs_link_up_1000basex() are almost identical
with the exception of checking the speed and duplex for 1000BASE-X.
Combine the two functions.

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>