Make TPM datagrams const, since they cannot be modified in the RO firmware.

firmware/stub/tpm_lite_stub.c

Index: firmware/stub/tpm_lite_stub.c
diff --git a/firmware/stub/tpm_lite_stub.c b/firmware/stub/tpm_lite_stub.c
index 3dc3dfead155b0298477a95efcd3a94103c50b0f..6df84645b34f48f5058b47692cb7ea1c5e200068 100644
--- a/firmware/stub/tpm_lite_stub.c
+++ b/firmware/stub/tpm_lite_stub.c
@@ -84,7 +84,7 @@ static void TpmExecute(const uint8_t *in, const uint32_t in_len,
 
 /* Gets the tag field of a TPM command.
  */
-POSSIBLY_UNUSED static INLINE int TpmTag(uint8_t* buffer) {
+POSSIBLY_UNUSED static INLINE int TpmTag(const uint8_t* buffer) {
   uint16_t tag;
   FromTpmUint16(buffer, &tag);
   return (int) tag;
@@ -129,7 +129,7 @@ void TlclOpenDevice(void) {
 }
 
 
-uint32_t TlclStubSendReceive(uint8_t* request, int request_length,
+uint32_t TlclStubSendReceive(const uint8_t* request, int request_length,
                              uint8_t* response, int max_length) {
   /*
    * In a real firmware implementation, this function should contain