modules/thingspeak: properly initialize mdata->node
modules/thingspeak: properly initialize mdata->node

thingspeak/update-channel was missing properly initialization
of mdata->node, what was causing failure of some packet
emissions.

Signed-off-by: Bruno Dilly <ea4fb6320c1221b6a559089d8ab97ffc53fc6586@intel.com>

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