Currently we may silently read/write a register at offset 0.  Change
this to WARN and then ignore the write or read-back all-ones.
Signed-off-by: NBen Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>