coresight: keep drvdata->size as the actual buffer size

Consistent with the what is done for TMC ETR, keep the
drvdata->size as representing the size of the actual buffer for
TMC ETF as well since size is used while dumping the buffer.

Change-Id: I2b5355a822898804add81c92bdd2992343195f2c
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>