coresight: graceful pcsave enable/disable for offline cores

Fail pcsave enable/disable if the core is offline so that it is
obvious to the user that the operation failed.

Change-Id: I2bd88636e1982a7de6b14f8b742d1f9bc7d23406
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>