thermal/of: Fix error code in of_thermal_zone_find()

Currently, if we cannot find the correct thermal zone then this error
path returns NULL and it would lead to an Oops in the caller.  Return
ERR_PTR(-EINVAL) instead.

Fixes: 3bd52ac87347 ("thermal/of: Rework the thermal device tree initialization")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YvDzovkMCQecPDjz@kili


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>