In 0826374b - i2c: Multiplexed I2C bus core support
core i2c code increased in size and complexity even when I2C_MUX
wasn't selected.

Turning this check into a constant NULL in the n case lets the
client functions in be simplified too, not needing to include
never-called calls to the mux-specific helpers.
Signed-off-by: NPhil Carmody <phil.carmody@partner.samsung.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>