ixgbe: set X550 MDIO speed before talking to PHY

ixgbe: set X550 MDIO speed before talking to PHY

The MDIO bus speed must be initialized before talking to the PHY the first
time in order to avoid talking to it using a speed that the PHY doesn't
support.

This fixes HW initialization error -17 (IXGBE_ERR_PHY_ADDR_INVALID) on
Denverton CPUs (a.k.a. the Atom C3000 family) on ports with a 10Gb network
plugged in. On those devices, HLREG0[MDCSPD] resets to 1, which combined
with the 10Gb network results in a 24MHz MDIO speed, which is apparently
too fast for the connected PHY. PHY register reads over MDIO bus return
garbage, leading to initialization failure.

Reproduced with Linux kernel 4.19 and 5.15-rc7. Can be reproduced using
the following setup:

* Use an Atom C3000 family system with at least one X552 LAN on the SoC
* Disable PXE or other BIOS network initialization if possible
  (the interface must not be initialized before Linux boots)
* Connect a live 10Gb Ethernet cable to an X550 port
* Power cycle (not reset, doesn't always work) the system and boot Linux
* Observe: ixgbe interfaces w/ 10GbE cables plugged in fail with error -17

Fixes: bcd6384d7d25 ("ixgbe: Introduce function to control MDIO speed")
Signed-off-by: Cyril Novikov <cnovikov@lynx.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>