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>

tools/testing/selftests/devices/probe/test_discoverable_devices.py

@@ -45,7 +45,7 @@ def find_pci_controller_dirs():
 
 
 def find_usb_controller_dirs():
-    usb_controller_sysfs_dir = "usb[\d]+"
+    usb_controller_sysfs_dir = r"usb[\d]+"
 
     dir_regex = re.compile(usb_controller_sysfs_dir)
     for d in os.scandir(sysfs_usb_devices):
@@ -91,7 +91,7 @@ def get_acpi_uid(sysfs_dev_dir):
 
 
 def get_usb_version(sysfs_dev_dir):
-    re_usb_version = re.compile("PRODUCT=.*/(\d)/.*")
+    re_usb_version = re.compile(r"PRODUCT=.*/(\d)/.*")
     with open(os.path.join(sysfs_dev_dir, "uevent")) as f:
         return int(re_usb_version.search(f.read()).group(1))