Link failure with CONFIG_SPL and CONFIG_I2C_MUX_PCA954x

Fix for the case of a U-Boot configuration with CONFIG_SPL,
CONFIG_I2C_MUX, CONFIG_I2C_MUX_PCA954x, no CONFIG_SPL_DM and no
CONFIG_SPL_I2C_MUX. Without this fix linking of pca954x fails because
dm_write_i2c does not exist because CONFIG_SPL_DM is not defined.

Signed-off-by: Kees Trommel <kees.trommel.contractor@draeger.com>
Cc:  Heiko Schocher <hs@denx.de>
diff --git a/drivers/i2c/muxes/Makefile b/drivers/i2c/muxes/Makefile
index 68ed9b5..b690821 100644
--- a/drivers/i2c/muxes/Makefile
+++ b/drivers/i2c/muxes/Makefile
@@ -2,6 +2,6 @@
 #
 # Copyright (c) 2015 Google, Inc
 obj-$(CONFIG_I2C_ARB_GPIO_CHALLENGE) += i2c-arb-gpio-challenge.o
-obj-$(CONFIG_$(SPL_)I2C_MUX) += i2c-mux-uclass.o
+obj-$(CONFIG_I2C_MUX) += i2c-mux-uclass.o
 obj-$(CONFIG_I2C_MUX_PCA954x) += pca954x.o
 obj-$(CONFIG_I2C_MUX_GPIO) += i2c-mux-gpio.o