tpm: Fix comparison of unsigned expression warning

The function tpm_xfer returns int so make 'err' be int rather than
uint32_t so that we can catch an error condition.  Reported by
clang-3.8.

Signed-off-by: Tom Rini <trini@konsulko.com>

lib/tpm.c

@@ -230,10 +230,9 @@ static uint32_t tpm_sendrecv_command(const void *command,
 		void *response, size_t *size_ptr)
 {
 	struct udevice *dev;
-	int ret;
+	int err, ret;
 	uint8_t response_buffer[COMMAND_BUFFER_SIZE];
 	size_t response_length;
-	uint32_t err;
 
 	if (response) {
 		response_length = *size_ptr;