kconfig: Remove duplicate call to sym_get_string_value()

Use the saved returned value of sym_get_string_value() instead of
calling it twice.

Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210215181511.2840674-2-mic@digikod.net
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/conf.c

@@ -137,7 +137,7 @@ static int conf_string(struct menu *menu)
 		printf("%*s%s ", indent - 1, "", menu->prompt->text);
 		printf("(%s) ", sym->name);
 		def = sym_get_string_value(sym);
-		if (sym_get_string_value(sym))
+		if (def)
 			printf("[%s] ", def);
 		if (!conf_askvalue(sym, def))
 			return 0;