of_mdio: use of_property_read_u32_array()

The "fixed-link" prop support predated of_property_read_u32_array(), so
basically had to open-code it. Using the modern API saves 24 bytes of the
object code (ARM gcc 4.8.5); the only behavior change would be that the
prop length check is now less strict (however the strict pre-check done
in of_phy_is_fixed_link() is left intact anyway)...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>