cr50: ensure the hash sizes are correct in U2F code
cr50: ensure the hash sizes are correct in U2F code

cr50 has CONFIG_UPTO_SHA512 defined, so the SHA digest size might be
larger than one might think ;-)
Set the right size for the operations using SHA-256 in the u2f code.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=cr50
BUG=b:35545754
TEST=pass U2FTest.

Change-Id: Ica71da5b9b61f324ce3af3652e9ff7a17fac3432
Reviewed-on: https://chromium-review.googlesource.com/558087
Commit-Ready: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@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