ARM: etm: Return the entire trace buffer if it is empty after reset

ARM: etm: Return the entire trace buffer if it is empty after reset

On some SOCs the read and write pointer are reset when the chip
resets, but the trace buffer content is preserved. If the status
bits indicates that the buffer is empty and we have never started
tracing, assume the buffer is full instead. This can be useful
if the system rebooted from a watchdog reset.

Change-Id: Iaf21c2c329c6059004ee1d38e3dfff66d7d28029
Signed-off-by: Arve Hjønnevåg <arve@android.com>