net: mii: remove mii_lpa_mod_linkmode_lpa_sgmii()

Vladimir points out that since we removed mii_lpa_to_linkmode_lpa_sgmii(),
mii_lpa_mod_linkmode_lpa_sgmii() is also no longer called.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>