Twinkie: update sniffer code
Twinkie: update sniffer code

- Remove hardware double-buffering : it's complex to get right and does
  not provide any performance improvement.
- Increase buffer size x2 to decrease overflow frequency.
- Fix buffering and simplify code.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BRANCH=none
BUG=chrome-os-partner:28337
TEST=make BOARD=twinkie
do a long acquisition on a sample pattern from the function generator
and verify that no packet are missing and the waveform looks good.

Reviewed-on: https://chromium-review.googlesource.com/223565
Reviewed-by: Todd Broch <tbroch@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
(cherry picked from commit ece9501f06e3eb5c0b09f440859f817d1d39a93c)
Signed-off-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

Change-Id: I12a9e8370d3f238e8894f15ce0190e2e0fbc26d7
Reviewed-on: https://chromium-review.googlesource.com/225684
Tested-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Reviewed-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Commit-Queue: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec