mii: remove mii_lpa_to_linkmode_lpa_sgmii()

The only caller of mii_lpa_to_linkmode_lpa_sgmii()
disappeared in v5.10.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>