kconfig: rename menu_get_parent_menu() to menu_get_menu_or_parent_menu()

The current menu_get_parent_menu() does not always return the parent
menu; if the given argument is itself a menu, it returns that menu.

Rename this function to better reflect this behavior.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>