tcpmv2: Handle discarded messages during EnterUSB
tcpmv2: Handle discarded messages during EnterUSB

Pending EnterUSB messages were not handling message discards. This
resulted in the EnterUSB message never transmitting if the port partner
sent a message while the EnterUSB message was queued.

BUG=b:168453520
BRANCH=none
TEST=make buildall
TEST=Connect Volteer to Gatkex board. Verify entry into USB4.

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: Ica45c098be3b8819aec3c8d3eeb5a8069f44dcb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2415081
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

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