usb_pd_protocol: Force rediscovering identity on boot
usb_pd_protocol: Force rediscovering identity on boot

This is useful with Apple's HDMI adapter, as the code that sends
the discovery message will also swap vconn as required.

BRANCH=none
BUG=chromium:644663
TEST=On elm, S5. Plug adapter with power+HDMI. Switch on elm,
     display works.

Change-Id: I21d47c69e2c7153a5d808dedcb1abe360ce3f5c0
Reviewed-on: https://chromium-review.googlesource.com/415698
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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