trace: ipc_logging: fix non-destructive read logic

When dumping logs through debugfs, a non-destructive read is performed
and after all data has been printed, the call waits on a completion for
new log data.  When that event is completed, the entire contents of the
logs are incorrectly printed.

Fix the read logic to correctly update the non-destructive read pointer
such that only the new data is printed to the logs.

CRs-Fixed: 681317
Change-Id: Ia12787b576b18b9f5174cefe91eb283979c286f7
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>