coresight: attempt to send data over STM only if size is non-zero

Send data over STM only if the data length specified is non-zero.
This will avoid potential issues while avoiding sending empty
packets.

Change-Id: I1792d4b342bdb8cae2c0a8edc2d2584b42c72f6f
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>