checkpatch: Allow checking more than one file.

checkpatch: Allow checking more than one file.

Currently to check more than one patch or file it's required
to invoke script for each file separately.
Fix that by iterating over all the passed filenames.

Note: If '-f' option passed, all the files treated as usual files.
      Without '-f' all the files treated as patch files.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>