net: phy: mdio-bcm-unimac: Fix clock handling

The DT binding for this PHY describes an *optional* clock property.
Due to a bug in the error handling logic, we are actually ignoring this
clock *all* of the time so far.

Fix this by using devm_clk_get_optional() to handle this clock properly.

Fixes: b78ac6ec

 ("net: phy: mdio-bcm-unimac: Allow configuring MDIO clock divider")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>