checkpatch: do not warn about -p0 patches when checking files

We are triggering the -p0 check for our own diffs generated using --file
command line option.  Suppress this check for files.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>