Update ssl3_get_channel_id for the simpler ECDSA_do_verify.

The return values are now 1/0, not 1/0/-1.

Change-Id: If65bb08a229c7944cb439ec779df461904d0ec19
Reviewed-on: https://boringssl-review.googlesource.com/1607
Reviewed-by: Adam Langley <agl@google.com>