scripts: checkpatch.pl: suppress warnings about MAINTAINERS

scripts: checkpatch.pl: suppress warnings about MAINTAINERS

The kernel expects changes to MAINTAINERS for all removed or added
files, printing warnings like:

  WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
  #828:
  deleted file mode 100644

Since this does not apply to "our" files in OpenWrt repo, this
warning should be disabled.

This can be achieved easiest by setting $reported_maintainer_file
to 1. While this is a hack that tricks the script into believing
the proper MAINTAINERS changes have been made, it's the easiest
solution as it does not require to touch any other code.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>