xhci: Fix use-after-free in xhci debugfs

Trying to read from debugfs after the system has resumed from
hibernate causes a use-after-free and thus a protection fault.

Steps to reproduce:
Hibernate system, resume from hibernate, then run
$ cat /sys/kernel/debug/usb/xhci/*/command-ring/enqueue

[ 3902.765086] general protection fault: 0000 [#1] PREEMPT SMP
...
[ 3902.765136] RIP: 0010:xhci_trb_virt_to_dma.part.50+0x5/0x30
...
[ 3902.765178] Call Trace:
[ 3902.765188]  xhci_ring_enqueue_show+0x1e/0x40
[ 3902.765197]  seq_read+0xdb/0x3a0
[ 3902.765204]  ? __handle_mm_fault+0x5fb/0x1210
[ 3902.765211]  full_proxy_read+0x4a/0x70
[ 3902.765219]  __vfs_read+0x23/0x120
[ 3902.765228]  vfs_read+0x8e/0x130
[ 3902.765235]  SyS_read+0x42/0x90
[ 3902.765242]  do_syscall_64+0x6b/0x290
[ 3902.765251]  entry_SYSCALL64_slow_path+0x25/0x25

The issue is caused by the xhci ring structures being reallocated
when the system is resumed, but pointers to the old structures
being retained in the debugfs files "private" field:

The proposed patch fixes this issue by storing a pointer to the xhci_ring
field in the xhci device structure in debugfs rather than directly
storing a pointer to the xhci_ring.

Fixes: 02b6fdc2 ("usb: xhci: Add debugfs interface for xHCI driver")
Signed-off-by: Alexander Kappner <agk@godking.net>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>