i2c: mux: make use of __i2c_smbus_xfer

Calling the __i2c_smbus_xfer wrapper in __i2c_mux_smbus_xfer provides
retries and thus makes the parent-locked case consistent with the both
mux-locked (i2c_mux_smbus_xfer) and the I2C transfer cases.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>