morello: Fix cppcheck violation - unused variable

The variable is only used for logging, so cppcheck is marking it as unused when the logging shouldn't occur
This commit fixes that problem only defining the variable when logging is enabled

Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>
Change-Id: I41c86f923610397f9bb75146b9b98abee2d08b08