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

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>