refactor: make cel_serial_read non-blocking

Drop timeout_ms; read now returns immediately with whatever
data is available (0 if none), so callers don't block the rest
of their loop waiting on serial I/O.

telemetry's poll loop now sleeps interval_ms itself between
empty reads via a small sleep_ms helper.

tools/telemetry.c

@@ -5,6 +5,12 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <time.h>
+#endif
+
 /* TX power index to dBm mapping (ELRS standard) */
 static int const s_tx_power_dbm[] = {
     0, 20, 26, 30, 32, 34, 36, 38,
@@ -31,6 +37,15 @@ static int telemetry_parse_link(int16_t* rssi, uint8_t* link_quality,
     return 0;
 }
 
+static void sleep_ms(int ms) {
+#ifdef _WIN32
+    Sleep((DWORD)ms);
+#else
+    struct timespec ts = {.tv_sec = ms / 1000, .tv_nsec = (ms % 1000) * 1000000L};
+    nanosleep(&ts, NULL);
+#endif
+}
+
 static void print_usage(char const* prog) {
     printf("Usage: %s --port <serial_port> [--baudrate <rate>] [interval_ms]\n", prog);
     printf("       %s --list\n", prog);
@@ -106,8 +121,11 @@ int main(int argc, char const* argv[]) {
     int frames = 0, errors = 0;
 
     for (int i = 0; i < 20; i++) { /* read up to 20 telemetry frames */
-        size_t n = cel_serial_read(port, buf, sizeof(buf), interval_ms);
-        if (n == 0) continue;
+        size_t n = cel_serial_read(port, buf, sizeof(buf));
+        if (n == 0) {
+            sleep_ms(interval_ms);
+            continue;
+        }
 
         cel_crsf_frame frame;
         if (cel_crsf_frame_parse(&frame, buf, n) != 0) {