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 <joe.lawrence@stratus.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-ext-caps.h

@@ -112,12 +112,16 @@ static inline int xhci_find_next_ext_cap(void __iomem *base, u32 start, int id)
 	offset = start;
 	if (!start || start == XHCI_HCC_PARAMS_OFFSET) {
 		val = readl(base + XHCI_HCC_PARAMS_OFFSET);
+		if (val == ~0)
+			return 0;
 		offset = XHCI_HCC_EXT_CAPS(val) << 2;
 		if (!offset)
 			return 0;
 	};
 	do {
 		val = readl(base + offset);
+		if (val == ~0)
+			return 0;
 		if (XHCI_EXT_CAPS_ID(val) == id && offset != start)
 			return offset;