Correct SPL uses of MULTIPLEXER

This converts 3 usages of this option to the non-SPL form, since there is
no SPL_MULTIPLEXER defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/mux.h b/include/mux.h
index 9f80991..c92d887 100644
--- a/include/mux.h
+++ b/include/mux.h
@@ -23,7 +23,7 @@
 struct udevice;
 struct mux_control;
 
-#if CONFIG_IS_ENABLED(MULTIPLEXER)
+#if IS_ENABLED(CONFIG_MULTIPLEXER)
 /**
  * mux_control_states() - Query the number of multiplexer states.
  * @mux: The mux-control to query.