Tests: USB: Decrease the max data size for iso ep

Although the USB spec sets the upper limit on FS isochronous endpoint
payloads to 1023 B, this value is hard to test in practice. Moreover,
not all the targets Mbed OS supports (like NUCLEO_F207ZG) are able to
handle all the endpoints set to max.

Author: Filip Jagodzinski

TESTS/usb_device/basic/USBEndpointTester.cpp

@@ -49,7 +49,19 @@
 #define TEST_SIZE_EP_INT_3      (16)
 #define TEST_SIZE_EP_INT_4      TEST_SIZE_EP_INT_MIN
 
-#define TEST_SIZE_EP_ISO_MAX    (1023)
+
+/* According to USB spec, the wMaxPacketSize for FS isochronous endpoints
+ * is 1023 B. There are a couple of reasons this value is not used in tests:
+ * - some of the boards supported by Mbed OS have too little RAM dedicated
+ *   for USB, making EndpointResolve::valid() fail when all the endpoints (2x
+ *   bulk, 2x interrupt, 2x isochronous, 2x control) are configured to use
+ *   the max value of wMaxPacketSize
+ *   (e.g. NUCLEO_F207ZG has 1.25K of endpoint RAM),
+ * - given a test host with other USB devices on the bus, it is unlikely
+ *   for the test device to be able to reserve the bandwidth associated with
+ *   high wMaxPacketSize for iso endpoints.
+ */
+#define TEST_SIZE_EP_ISO_MAX    (256)
 #define TEST_SIZE_EP_ISO_MIN    (1)
 #define TEST_SIZE_EP_ISO_0      (0)
 #define TEST_SIZE_EP_ISO_1      (0)