coresight: enable flush-on-flushin for periodic flush to usb

Periodic flushing for ETR to USB uses ETR flush-on-FLUSHIN
external input. Hence, enable flush-on-FLUSHIN by default when
enabling ETR to USB transfers and work around the manual flush
failure by skipping it during ETR to USB disable. Adjust the
periodic flush threshold to the maximum value in order to
reduce the overhead when there is no data to be transferred.

CRs-Fixed: 461885
Change-Id: Ie3a1bbc80e017f187af56fcfdfb52297ba9aa72d
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>