Remove the set_speed function and allow the driver to figure out the
speed at which needs to configure the serdes based on the interface type.

Fixes: 30552490 ("phy: Add lan966x ethernet serdes PHY driver")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20211211214717.1284306-1-horatiu.vultur@microchip.comSigned-off-by: Vinod Koul <vkoul@kernel.org>