scripts: get_maintainer: Fix glob pattern directory match error logic

The `get_maintainer` script currently generates an error when a non-
slash-suffixed glob pattern matches any directories -- this is
incorrect because the glob pattern may be used to match files.

This commit updates the script to only generate an error when a
non-slash suffixed _only_ matches directories.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>