fixdep: use existing helper to check modular CONFIG options

str_ends_with() tests if the given token ends with a particular string.
Currently, it is used to check file paths without $(srctree).

Actually, we have one more place where this helper is useful.  Use it
to check if CONFIG option ends with _MODULE.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>