usb: xhci: bail out of setup if the controller is inaccessible

In the Linux kernel, the following vulnerability has been resolved:

usb: xhci: bail out of setup if the controller is inaccessible

xhci_gen_setup() locates the operational registers using the capability
length read from the very first register:

xhci->op_regs = hcd->regs +
HC_LENGTH(readl(&xhci->cap_regs->hc_capbase));

If the controller is dead or has dropped off the bus, that read returns
~0, HC_LENGTH() truncates it to 0xff, and op_regs ends up 0xff bytes
past the page-aligned MMIO base, i.e. unaligned. The first access
through it, xhci_halt() -> xhci_handshake() reading op_regs->status, is
then an unaligned readl() on device memory. arm64 faults on unaligned
device accesses, so instead of xhci_handshake() catching the all-ones
value and returning -ENODEV, setup oopses:

xhci-pci-renesas 0005:08:00.0: Unable to change power state from D3cold to D0, device inaccessible
xhci-pci-renesas 0005:08:00.0: xHCI Host Controller
xhci-pci-renesas 0005:08:00.0: new USB bus registered, assigned bus number 1
Unable to handle kernel paging request at virtual address ffff80030a770103
ESR = 0x0000000096000021
FSC = 0x21: alignment fault
Internal error: Oops: 0000000096000021 [#1] SMP
pc : xhci_halt [xhci_hcd]
Call trace:
xhci_halt
xhci_gen_setup
xhci_pci_setup
usb_add_hcd
usb_hcd_pci_probe
xhci_pci_common_probe
xhci_pci_renesas_probe

This was hit with a Renesas uPD720201 that failed to power up ("Unable
to change power state from D3cold to D0, device inaccessible") yet still
reached the HCD probe path.

Read the capability register once, and if it reads back the all-ones
value (as xhci_handshake() and xhci_reset() already test for), abort
setup with -ENODEV before op_regs is derived from it. Reading it once
also avoids re-reading a register that may change under a concurrent
hot-removal.