USB: serial: export usb_serial_generic_write_start

Export usb_serial_generic_write_start which is needed when implementing
a custom resume function while still relying on the generic write
implementation.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/usb/serial.h b/include/linux/usb/serial.h
index d528b80..704a1ab 100644
--- a/include/linux/usb/serial.h
+++ b/include/linux/usb/serial.h
@@ -320,6 +320,8 @@
 extern void usb_serial_put(struct usb_serial *serial);
 extern int usb_serial_generic_open(struct tty_struct *tty,
 	struct usb_serial_port *port);
+extern int usb_serial_generic_write_start(struct usb_serial_port *port,
+							gfp_t mem_flags);
 extern int usb_serial_generic_write(struct tty_struct *tty,
 	struct usb_serial_port *port, const unsigned char *buf, int count);
 extern void usb_serial_generic_close(struct usb_serial_port *port);