coresight: fix cti gate enable disable apis functionalities

Setting the CoreSight CTI Gate register enables CTI channel
propagation and clearing this register disables channel
propagation. Current implementation of CTI gate enable and
disable apis is not consistent with this channel propagation
behavior. Fix these api implementaions.

Change-Id: Ibe93f97a785c6c33753c18db4417997f9fbf5af1
Signed-off-by: Aparna Das <adas@codeaurora.org>