gsctool: improve vendor command processing robustness
gsctool: improve vendor command processing robustness

With upcoming RMA authentication extensions the size of the vendor
command responses is going to increase. Let's allow 500 bytes per
response (the expected maximum is 80 bytes plus TPM header).

BRANCH=cr50
BUG=b:37952913
TEST=verified that gsctool (aka usb_updater) still allows to retrieve
     Cr59 information and update Cr50 image.

Change-Id: Ic61b6b89ffe20e534029bd12fea4140882a9afc8
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/690442
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

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