net: axienet: set mdio clock according to bus-frequency

Some FPGA platforms have 80KHz MDIO bus frequency constraint when
connecting Ethernet to its on-board external Marvell PHY. Thus, we may
have to set MDIO clock according to the DT. Otherwise, use the default
2.5 MHz, as specified by 802.3, if the entry is not present.

Also, change MAX_MDIO_FREQ to DEFAULT_MDIO_FREQ because we may actually
set MDIO bus frequency higher than 2.5MHz if undelying devices support
it. And properly disable the mdio bus clock in error path.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>