fs/archelp: If path given to grub_archelp_dir() is not a directory return error

fs/archelp: If path given to grub_archelp_dir() is not a directory return error

Specifically, return GRUB_ERR_BAD_FILE_TYPE because this is what is
expected by the ls command when it is given a path to a non-directory.
This fixes a bug where calling ls with a list of non-directory paths
outputs a blank line for each such argument.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>