coresight: ignore multiple calls to coresight_abort

coresight_abort allows for abruptly stopping the current sink from
accepting new data hence avoiding polluting trace with information
after the point of interest.

Depending upon the call site, coresight_abort could end up getting
called more than once. This change will help with ignoring
subsequent calls to coresight_abort so that abort callback is
called just once. This is in line with treating abort as a one time
abrupt stop.

Change-Id: I634001e832ee7fa1b23f67698bd2d5dd12f8d65d
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>