USB: serial: generalise generic read implementation

Add process_read_urb to usb_serial_driver so that a driver can rely on
the generic read (and throttle) mechanism but still do device specific
processing of incoming data (such as adding tty_flags before pushing to
line discipline).

The default generic implementation handles sysrq for consoles but
otherwise simply pushes to tty.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 5389246..1b92442 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -1298,6 +1298,7 @@
 	set_to_generic_if_null(device, write_bulk_callback);
 	set_to_generic_if_null(device, disconnect);
 	set_to_generic_if_null(device, release);
+	set_to_generic_if_null(device, process_read_urb);
 }
 
 int usb_serial_register(struct usb_serial_driver *driver)