dfu:tests: Modify dfu_gadget_test.sh to accept USB device vendor:product ID

dfu-util allows filtering on USB device vendor:product ID by using
the -d flag (-d 0451:d022).
Such option is very handy when many DFU devices are connected to a single
host PC. This commit allows testing when above situation emerges.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>

Tested-by: Lukasz Majewski <l.majewski@majess.pl>
Test HW - AM335x Beagle Bone Black
NOTE: Max size of file to transfer: 2MiB
diff --git a/test/dfu/README b/test/dfu/README
index 5176aba..408d559 100644
--- a/test/dfu/README
+++ b/test/dfu/README
@@ -26,12 +26,19 @@
    setenv dfu_alt_info dfu_test.bin fat 0 6\;dfudummy.bin fat 0 6
    dfu 0 mmc 0
 2. On the host:
-   test/dfu/dfu_gadget_test.sh X Y  [test file name]
+   test/dfu/dfu_gadget_test.sh X Y [test file name] [usb device vendor:product]
    e.g. test/dfu/dfu_gadget_test.sh 0 1
    or
    e.g. test/dfu/dfu_gadget_test.sh 0 1 ./dat_960.img
+   or
+   e.g. test/dfu/dfu_gadget_test.sh 0 1 0451:d022
+   or
+   e.g. test/dfu/dfu_gadget_test.sh 0 1 ./dat_960.img 0451:d022
 
 ... where X and Y are dfu_test.bin's and dfudummy.bin's alt setting numbers.
 They can be obtained from dfu-util -l or $dfu_alt_info.
 It is also possible to pass optional [test file name] to force the script to
 test one particular file.
+If many DFU devices are connected, it may be useful to filter on USB
+vendor/product ID (0451:d022).
+One can get them by running "lsusb" command on a host PC.