i2c: add missing of_node_put in i2c_mux_del_adapters

Refcount of of_node is increased with of_node_get() in i2c_mux_add_adapter().
It must be decreased with of_node_put() in i2c_mux_del_adapters().

Cc: stable@vger.kernel.org
Signed-off-by: Qi Hou <qi.hou@windriver.com>
Reviewed-by: Zhang Xiao <xiao.zhang@windriver.com>
Signed-off-by: Peter Rosin <peda@axentia.se>