usb: dwc3: st: Add of_node_put() before return in probe function

The local variable child in the function st_dwc3_probe takes the return
value of of_get_child_by_name, which gets a node and does not put it. If
the function returns without releasing child, this could cause a memory
error. Hence put child as soon as there is no more use for it. Also
create a new label, err_node_put, just before label undo_softreset; so
that err_node_put puts child. In between initialisation of child and its
first put, modify all statements that go to undo_softreset to now go to
err_node_put instead, from where they can fall through to
undo_softreset.
Issue found with Coccinelle.

Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>