xhci: harden xhci_find_next_ext_cap against device removal

xhci: harden xhci_find_next_ext_cap against device removal

xhci_find_next_ext_cap doesn't check for PCI hotplug removal and may use
the PCI master abort bit pattern (~0) to calculate a new PCI address
offset to read/write.  The has lead to reproducable crashes when testing
surprise removal during device initialization on a Stratus platform, at
least after commit d5ddcdf4d672 ("xhci: rework xhci extended capability
list parsing functions").

The crash is repeatable on a Stratus platform when injecting hardware
faults to induce xHCI host controller hotplug during driver
initialization.  If a PCI read in xhci_find_next_ext_cap returns the
master abort pattern, quirk_usb_handoff_xhci may start using a bogus
ext_cap_offset to start searching more bogus PCI addresses.

Signed-off-by: Joe Lawrence <[email protected]>
Acked-by: Mathias Nyman <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>