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/drivers/mux/Makefile b/drivers/mux/Makefile
index 78ebf04..d4e2478 100644
--- a/drivers/mux/Makefile
+++ b/drivers/mux/Makefile
@@ -3,5 +3,5 @@
 # (C) Copyright 2019
 # Jean-Jacques Hiblot <jjhiblot@ti.com>
 
-obj-$(CONFIG_$(SPL_)MULTIPLEXER) += mux-uclass.o
+obj-$(CONFIG_MULTIPLEXER) += mux-uclass.o
 obj-$(CONFIG_$(SPL_)MUX_MMIO) += mmio.o