Make sure to check the tty-device pointer before looking up the sibling
platform device to avoid dereferencing a NULL-pointer when the tty is
one end of a Unix98 pty.

Fixes: 74cdad37 ("Bluetooth: hci_intel: Add runtime PM support")
Fixes: 1ab1f239 ("Bluetooth: hci_intel: Add support for platform driver")
Cc: stable <stable@vger.kernel.org>     # 4.3
Cc: Loic Poulain <loic.poulain@intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>