USB: serial: sierra.c: use module_usb_serial_driver

USB: serial: sierra.c: use module_usb_serial_driver

This converts the sierra.c driver to use the module_usb_serial_driver() call
instead of having to have a module_init/module_exit function, saving a lot
of duplicated code.

CC: Alan Stern <stern@rowland.harvard.edu>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Johan Hovold <jhovold@gmail.com>
CC: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>