If the directory path given to INSTALL_MOD_PATH contains % or :,
the module_install fails.

% is used in pattern rules, and : as the separator of dependencies.

Bail out with a clearer error message.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NNicolas Schier <n.schier@avm.de>