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 --cc drivers/usb/serial/generic.c
Simple merge

diff --cc include/linux/usb/serial.h
Simple merge