byte-cntr: Set offset as rwp offset when sw usb mode start

Set offset as rwp offset when sw usb mode start to avoid the data
transfer delay issue. If use etr_buf offset as initial offset,
there will be interval between rwp offset and initial offset.
Then the delay will happen.

Change-Id: I92b4326a4c38384f68bd0c60f3b487d80b334325
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>