usb-pd: debounce vbus detection after PR_SWAP
usb-pd: debounce vbus detection after PR_SWAP

Vbus is supposed to be present when the PS_RDY PD command is sent by the
new source, but if an external TCPC debounces the Vbus detection
(which is worth while) and does not debounce the RX messages (debounce
isn't needed), then it looks like we get PS_RDY before Vbus is presence
which cause the PD state machine to disconnect (which is also the proper
thing to do when acting as a SNK and VBus isn't present).

Add a Vbus debounce time that allows us to ignore the absence of Vbus
for a small window after receiving the PD_RDY packet from the new SRC
after performing a Power Role SWAP (PR_SWAP).

BRANCH=none
BUG=b:112451526
TEST=Tested with a monitor that sends PS_RDY and asserts Vbus within 1ms of
each other; this caused issue before and now it connects.

Change-Id: I0ebfe04d97a15298ca3a5e7e5a3b23b4ab2edfbf
Signed-off-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1238081
Reviewed-by: Edward Hill <db1406b34e091703092b6b136bdfeb2c8125fc0e@chromium.org>
Reviewed-by: Nitin Kolluru <bc2d92dd98caa1d939292891a48c5ccc55a8a8d0@google.com>

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