USB: xhci: dbc: fix tty registration race

USB: xhci: dbc: fix tty registration race

Make sure to allocate resources before registering the tty device to
avoid having a racing open() and write() fail to enable rx or
dereference a NULL pointer when accessing the uninitialised fifo.

Fixes: dfba2174dc42 ("usb: xhci: Add DbC support in xHCI driver")
Cc: stable@vger.kernel.org      # 4.16
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20211008092547.3996295-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>