Bluetooth: Fix hci_suspend_sync crash

If hci_unregister_dev() frees the hci_dev object but hci_suspend_notifier
may still be accessing it, it can cause the program to crash.
Here's the call trace:
  <4>[102152.653246] Call Trace:
  <4>[102152.653254]  hci_suspend_sync+0x109/0x301 [bluetooth]
  <4>[102152.653259]  hci_suspend_dev+0x78/0xcd [bluetooth]
  <4>[102152.653263]  hci_suspend_notifier+0x42/0x7a [bluetooth]
  <4>[102152.653268]  notifier_call_chain+0x43/0x6b
  <4>[102152.653271]  __blocking_notifier_call_chain+0x48/0x69
  <4>[102152.653273]  __pm_notifier_call_chain+0x22/0x39
  <4>[102152.653276]  pm_suspend+0x287/0x57c
  <4>[102152.653278]  state_store+0xae/0xe5
  <4>[102152.653281]  kernfs_fop_write+0x109/0x173
  <4>[102152.653284]  __vfs_write+0x16f/0x1a2
  <4>[102152.653287]  ? selinux_file_permission+0xca/0x16f
  <4>[102152.653289]  ? security_file_permission+0x36/0x109
  <4>[102152.653291]  vfs_write+0x114/0x21d
  <4>[102152.653293]  __x64_sys_write+0x7b/0xdb
  <4>[102152.653296]  do_syscall_64+0x59/0x194
  <4>[102152.653299]  entry_SYSCALL_64_after_hwframe+0x5c/0xc1

This patch holds the reference count of the hci_dev object while
processing it in hci_suspend_notifier to avoid potential crash
caused by the race condition.

Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>