test/py: usb: Fix format string for fstype command

USB tests on ext partitions can fail with the following output

test/py/tests/test_usb.py:245: in test_usb_part
    'fstype usb %d:%d' % i, part_id
E   TypeError: not enough arguments for format string

So add brackets around the format string arguments to prevent the
error.

Fixes: a730947974e3 ("test/py: usb: Distinguish b/w ext2/ext4 partitions")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Love Kumar <love.kumar@amd.com>
diff --git a/test/py/tests/test_usb.py b/test/py/tests/test_usb.py
index e1f203b..566d73b 100644
--- a/test/py/tests/test_usb.py
+++ b/test/py/tests/test_usb.py
@@ -242,7 +242,7 @@
                     elif part_type == '83':
                         print('ext(2/4) detected')
                         output = u_boot_console.run_command(
-                            'fstype usb %d:%d' % i, part_id
+                            'fstype usb %d:%d' % (i, part_id)
                         )
                         if 'ext2' in output:
                             part_ext2.append(part_id)