common:pd: reset CapCnt variable
common:pd: reset CapCnt variable

CapCnt variable will be reset in PD_STATE_SRC_STARTUP,
PD_STATE_SRC_DISCOVERY and PD_STATE_SNK_SWAP_COMPLETE states
which are according to PD3.0 spec. With this modify in order to
protect against future changes that allow other code path
that don't clear the variables first.

BUG=none
BRANCH=none
TEST=make buildall -j.

Change-Id: Ic2a249aff4410043ab18dd645aa1485ece9f63ca
Signed-off-by: Ruibin Chang <6748d95250dd3d29f04d200035d5434f3b7820ef@ite.com.tw>
Reviewed-on: https://chromium-review.googlesource.com/1195169
Reviewed-by: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

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