Bluetooth: hci_bcm: add missing tty-device sanity check

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: 0395ffc1 ("Bluetooth: hci_bcm: Add PM for BCM devices")
Cc: stable <stable@vger.kernel.org>     # 4.3
Cc: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>