nrf/drivers/usb: Add a tud_cdc_rx_cb() callback to check interrupt char.

ports/nrf/drivers/usb/usb_cdc.c

@@ -46,7 +46,7 @@
 
 extern void tusb_hal_nrf_power_event(uint32_t event);
 
-static void cdc_task(void);
+static void cdc_task(bool tx);
 
 static uint8_t rx_ringbuf_array[1024];
 static uint8_t tx_ringbuf_array[1024];
@@ -121,7 +121,7 @@ static int cdc_tx_char(void) {
     return ringbuf_get((ringbuf_t*)&tx_ringbuf);
 }
 
-static void cdc_task(void)
+static void cdc_task(bool tx)
 {
     if ( tud_cdc_connected() ) {
         // connected and there are data available
@@ -138,24 +138,30 @@ static void cdc_task(void)
             }
         }
 
-        int chars = 0;
-        while (cdc_tx_any()) {
-            if (chars < 64) {
-               tud_cdc_write_char(cdc_tx_char());
-               chars++;
-            } else {
-               chars = 0;
-               tud_cdc_write_flush();
+        if (tx) {
+            int chars = 0;
+            while (cdc_tx_any()) {
+                if (chars < 64) {
+                    tud_cdc_write_char(cdc_tx_char());
+                    chars++;
+                } else {
+                    chars = 0;
+                    tud_cdc_write_flush();
+                }
             }
-        }
 
-        tud_cdc_write_flush();
+            tud_cdc_write_flush();
+        }
     }
 }
 
 static void usb_cdc_loop(void) {
     tud_task();
-    cdc_task();
+    cdc_task(true);
+}
+
+void tud_cdc_rx_cb(uint8_t itf) {
+    cdc_task(false);
 }
 
 int usb_cdc_init(void)