kselftest/devices/probe: Fix SyntaxWarning in regex strings for Python3

Insert raw strings to prevent Python3 from interpreting string literals
as Unicode strings and "\d" as invalid escaped sequence.

Fix the warnings:

tools/testing/selftests/devices/probe/test_discoverable_devices.py:48:
SyntaxWarning: invalid escape sequence '\d' usb_controller_sysfs_dir =
"usb[\d]+"

tools/testing/selftests/devices/probe/test_discoverable_devices.py: 94:
SyntaxWarning: invalid escape sequence '\d' re_usb_version =
re.compile("PRODUCT=.*/(\d)/.*")

Fixes: dacf1d7a ("kselftest: Add test to verify probe of devices from discoverable buses")

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>