i2c: mux-gpmux: Add of_node_put() when breaking out of loop

In i2c_mux_probe(), we should call of_node_put() when breaking out
of for_each_child_of_node() which will automatically increase and
decrease the refcount.

Fixes: ac8498f0 ("i2c: i2c-mux-gpmux: new driver")
Signed-off-by: Liang He <windhl@126.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>