agilent-dmm: Make serial write call block in send.

This call is executed from an event handler context was previously
nonblocking, however there is no handling for a partial write.

The output buffer is unlikely to be full and the commands to be sent
are short, so it should be OK to make this a blocking call.

src/hardware/agilent-dmm/sched.c

@@ -140,7 +140,7 @@ static int agdmm_send(const struct sr_dev_inst *sdi, const char *cmd)
 		strncat(buf, "\r\n", 32);
 	else
 		strncat(buf, "\n\r\n", 32);
-	if (serial_write(serial, buf, strlen(buf)) == -1) {
+	if (serial_write_blocking(serial, buf, strlen(buf)) == -1) {
 		sr_err("Failed to send: %s.", strerror(errno));
 		return SR_ERR;
 	}