Bugfix: validation always fails: 'str' object has no attribute 'get'
Bugfix: validation always fails: 'str' object has no attribute 'get'

Recently changed data validation function does its checks by treating
attributes/values as they were dictionaries; however, they are lists
of dictionaries.

data["telemetry"].get("ts") is bound to fail: the correct check would be
data["telemetry"][i].get("ts"), for each element i.

Signed-off-by: Simone Libutti <7dbd3541d180fb2293c2d424b6ba631d047e4733@ibtsystems.it>

thingsboard/thingsboard-gateway,thingsboard/thingsboard-gateway