tpm2_tlcl: Use signed integer for tpm2_marshal_command return value

The tpm2_marshal_command() function returns a negative value on error,
so we must use a signed type for the return value.

This was found by the coverity scan:
https://scan.coverity.com/projects/coreboot?tab=overview
CID:1357675
CID:1357676

Change-Id: I56d2ce7d52b9b70e43378c13c66b55ac2948f218
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Found-by: Coverity Scan
Reviewed-on: https://review.coreboot.org/15717
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

src/lib/tpm2_tlcl.c

@@ -21,7 +21,8 @@
 
 static void *tpm_process_command(TPM_CC command, void *command_body)
 {
-	size_t out_size, in_size;
+	ssize_t out_size;
+	size_t in_size;
 	/* Command/response buffer. */
 	static uint8_t cr_buffer[TPM_BUFFER_SIZE];