cifs: clean up checks in cifs_echo_request

Follow-on patch to 7e90d705

 which is already in Steve's tree...

The check for tcpStatus == CifsGood is not meaningful since it doesn't
indicate whether the NEGOTIATE request has been done. Also, clarify
why we're checking for maxBuf == 0.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>