microblaze: intc: Reformat output

A message was using pr_info level output for a message including "ERROR"
which is not really a fatal error.
Remove the 'ERROR' from that message, use pr_warn loglevel and add the
function name to the output to give users a chance to find the culprit
in case the warning triggers.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>