spdm: Fix requester session logic

Fix 2 bugs in the requester:
1. The size of vendor request doesn't add the vendor request header size
2. We mistakenly returns an error when the session is established
   successfully.

BUG=b:338528575
TEST=spdm unittests

Change-Id: I3fb1354cfde3756919ac9800b0c9532fe6d07366
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/spdm/+/5537348
Reviewed-by: Vadim Sukhomlinov <sukhomlinov@google.com>
Tested-by: Howard Yang <hcyang@google.com>
Commit-Queue: ChromeOS Auto Retry <chromeos-auto-retry@chromeos-bot.iam.gserviceaccount.com>
2 files changed