dm: usb: Clean up USB after each test

The USB subsystem has a few counters that need to be reset since they are
stored in static variables rather than driver-model data. An example is
usb_max_devs. Ultimately we should move this data into the USB uclass.

For now, make sure that USB is reset after each test, so that the counters
go back to zero.

Note: this is not a perfect solution: It a USB test fails it will exit
immediately and leave USB un-reset. The impact here is that it may cause
subsequence test failures in the same run.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/usb.c

@@ -52,6 +52,7 @@ static int dm_test_usb_flash(struct unit_test_state *uts)
 	memset(cmp, '\0', sizeof(cmp));
 	ut_asserteq(2, blk_dread(dev_desc, 0, 2, cmp));
 	ut_assertok(strcmp(cmp, "this is a test"));
+	ut_assertok(usb_stop());
 
 	return 0;
 }
@@ -67,6 +68,7 @@ static int dm_test_usb_multi(struct unit_test_state *uts)
 	ut_assertok(uclass_get_device(UCLASS_MASS_STORAGE, 0, &dev));
 	ut_assertok(uclass_get_device(UCLASS_MASS_STORAGE, 1, &dev));
 	ut_assertok(uclass_get_device(UCLASS_MASS_STORAGE, 2, &dev));
+	ut_assertok(usb_stop());
 
 	return 0;
 }