usb: gadget: serial: fix oops when data rx'd after close

When the gadget serial device has no associated TTY, do not pass any
received data into the TTY layer for processing; simply drop it instead.
This prevents the TTY layer from calling back into the gadget serial
driver, which will then crash in e.g. gs_write_room() due to lack of
gadget serial device to TTY association (i.e. a NULL pointer dereference).

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>