TCPMv2: Return to ready state after PE_VDM_ACKED
TCPMv2: Return to ready state after PE_VDM_ACKED

Facilitate gradual migration from PE_DO_PORT_DISCOVERY to other
functions for mode entry. The PE should return to the appropriate ready
state between each VDM send/receive cycle to more closely comport with
the PD spec.

BUG=b:155890173
TEST=Attach a dongle; state flows into and out of PE_SRC_Ready between
     Enter Mode, DP Status, and DP Configure flows; message flow
     observed via Twinkie unchanged.
BRANCH=none

Change-Id: I839dc835ce1216321aa9be75a3af4447503fb083
Signed-off-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2217060
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>

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