Calling 'mdio read ...' currently leads to a data abort when no mdio
bus is found.
To fix this, check if 'bus' is a valid pointer before accessing it.
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com> Tested-by: Vladimir Oltean <olteanv@gmail.com> Reviewed-by: Vladimir Oltean <olteanv@gmail.com> Acked-by: Joe Hershberger <joe.hershberger@ni.com>