kernel: xhci: fix USB3 port events with a single roothub

kernel: xhci: fix USB3 port events with a single roothub

USB3 port events are dropped whenever the controller has only one roothub.
The check that guards them treats a missing shared hcd as proof the hcd was
removed, which stopped being true with upstream commit 4736ebd7fcaf ("usb:
host: xhci-plat: omit shared hcd if either root hub has no ports"). A
controller whose USB2 root hub has no ports gets a single roothub and never
has a shared one, so every SuperSpeed event is thrown away and nothing
enumerates even though the port reports the device:

  0x006a1203 Powered Connected Enabled Link:U0 PortSpeed:4

Broadcom Northstar is such a controller. USB3 works there up to 5.15 and
stops from 5.19 onwards.

Tested on an Asus RT-N18U with kernel 6.18, seven SuperSpeed enumerations,
USB2 unaffected on both ports.

Link: https://github.com/openwrt/openwrt/issues/19592
Signed-off-by: Semih Baskan <strst.gs@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24547
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>