i2c: muxes: mule: Fix an error handling path in mule_i2c_mux_probe()

If an error occurs in the loop that creates the device adapters, then a
reference to 'dev' still needs to be released.

Use for_each_child_of_node_scoped() to both fix the issue and save one line
of code.

Fixes: d0f8e978 ("i2c: muxes: add support for tsd,mule-i2c multiplexer")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>