phy: qcom-sgmii-eth: remove .set_speed() implementation

phy: qcom-sgmii-eth: remove .set_speed() implementation

Now that the qcom-ethqos driver has migrated to use phy_set_mode_ext()
rather than phy_set_speed() to configure the SerDes, the support for
phy_set_speed() is now obsolete. Remove support for this method.

Using the MAC speed for the SerDes is never correct due to the PCS
encoding. For SGMII and 2500BASE-X, the PCS uses 8B10B encoding, and
so:

  MAC rate * PCS output bits / PCS input bits = SerDes rate
   1000M   *       10        /       8        = 1250M
   2500M   *       10        /       8        = 3125M

Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vxS49-0000000BQXa-3Zcg@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>