net: of_mdio: don't store the length of a property if we don't need to

of_get_property() can be called with NULL as 2nd argument if the caller
is not interested in the length of a property. Use that here so we can
get rid of a variable.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>