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>