i2c: mux: mlxcpld: make use of __i2c_smbus_xfer

This simplifies the code, and you get retries for free if the adapter
does not support ->master_xfer.

Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Michael Shych <michaelsh@mellanox.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>