net: pcs: xpcs: rename xpcs_config_usxgmii()

xpcs_config_usxgmii() is only called from the xpcs_link_up() method, so
let's name it similarly to the SGMII and 1000BASEX 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>