tcpm: fusb302: Round VNC and Rd thresholds to nearest step
tcpm: fusb302: Round VNC and Rd thresholds to nearest step

fusb302 determines attach / no-attach (and Rd / Ra) by comparing CC
voltage against an MDAC output (42 mV steps). The previous 'floor'
calculation was particularly bad for 3.0A Rp (2600 / 42 = 61, 61 * 42 =
2562 mV - 21 = 2551 mV actual threshold, ignoring other error sources).
Reduce the chance of error by rounding our thresholds, which also
matches the suggested thresholds in the datasheet.

BUG=chromium:758608
BRANCH=gru
TEST=Attach problematic dingdong, verify we don't enter an attach /
detach loop.

Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Change-Id: I9211782da0fdad8339246e272952ba1930b69851
Reviewed-on: https://chromium-review.googlesource.com/633276
Reviewed-by: Joe Bauman <08b3f82825161d48ab8fa058be63dcf42101e942@fairchildsemi.com>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
(cherry picked from commit 311b3e4e15fd37ea2ab151edb8b8a468e93355fd)
Reviewed-on: https://chromium-review.googlesource.com/638694
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec