powercap: Adjust printing the constraint name with new line

The constrain name has limit of size 30, which sometimes might be hit.
When this happens the new line might get lost. Prevent this and set the
max limit for name string length equal 29. This would result is proper
string clamping (when needed) and storing '\n' at index 29 and '\0' at 30,
so similarly as desired originally.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>