cli: Drop #ifdefs for CONFIG_AUTO_COMPLETE in cli_readline

Use a static inline and adjust the logic to avoid the need for #ifdefs in
cli_readline_into_buffer()

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/command.h b/include/command.h
index 34ea989..1c4ec42 100644
--- a/include/command.h
+++ b/include/command.h
@@ -95,6 +95,12 @@
 		 char *cmdv[]);
 int cmd_auto_complete(const char *const prompt, char *buf, int *np,
 		      int *colp);
+#else
+static inline int cmd_auto_complete(const char *const prompt, char *buf,
+				    int *np, int *colp)
+{
+	return 0;
+}
 #endif
 
 /**