usb-device-cdc: Fix short delays when timeout is 0.

Previously even with timeout set to zero, a read could block
for up to 1ms + machine.idle() timeout period.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

micropython/usb/usb-device-cdc/usb/device/cdc.py

@@ -367,9 +367,11 @@ class CDCInterface(io.IOBase, Interface):
             mv = mv[nbytes:]
 
             # check for timeout
-            if time.ticks_diff(time.ticks_ms(), start) > self._timeout:
+            if time.ticks_diff(time.ticks_ms(), start) >= self._timeout:
                 return len(buf) - len(mv)
 
+            machine.idle()
+
     def read(self, size):
         start = time.ticks_ms()
 
@@ -404,7 +406,7 @@ class CDCInterface(io.IOBase, Interface):
                 if n == len(b):
                     break  # Done, exit before we reach the sleep
 
-            if time.ticks_diff(time.ticks_ms(), start) > self._timeout:
+            if time.ticks_diff(time.ticks_ms(), start) >= self._timeout:
                 break  # Timed out
 
             machine.idle()