scmi_sensor: Fix return value from response event

This patch fixes the status returned to scmi when
the result is provided through a response event. In the
case of a hardware failure, the agent is now correctly
informed of such state.

Change-Id: I96c09e3f63afa86bef64da81da346895ea738f22
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>