i2c: mux: mlxcpld: simplify code to reach the adapter

struct i2c_client has a direct pointer to the adapter, no need to dig
it out of the struct device tree.

Suggested-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Peter Rosin <peda@axentia.se>