Properly configure USB 3.1 speed, if supported by the kernel

USBIP in Linux 6.12 accepts USB_SPEED_SUPER_PLUS now (see
bb548c1654db4a6f1a42657ed9068237e0c044b5 in Linux). Configure USB 3.1
device this way if kernel is new enough.

QubesOS/qubes-issues#9587

src/usb-import

@@ -125,13 +125,18 @@ if [ -z "$untrusted_devid" ] && [ -z "$untrusted_speed" ]; then
     exit 1
 fi
 
+max_supported_speed=5
+if printf "6.12\n%s\n" "$(uname -r)" | sort -VC; then
+    max_supported_speed=6
+fi
+
 case "$untrusted_speed" in
     1.5) speed=1 ;; # Low Speed
     12)  speed=2 ;; # Full speed
     480) speed=3 ;; # High Speed
     53.3-480) speed=4 ;; # Wireless
     5000) speed=5 ;; # Super Speed
-    10000) speed=5 ;; # Super Speed Plus (USB 3.1); Announce as USB 3.0 until USBIP get support
+    10000) speed=$max_supported_speed ;; # Super Speed Plus (USB 3.1); Announce as USB 3.0 until USBIP get support
     *) ERROR "Invalid speed \"$untrusted_speed\" received." \
              "Expected \"1.5\", \"12\", \"480\", \"53.3-480\", \"5000\", \"10000\". " \
              "If the remote side sent nothing, this could mean "\