USB: serial: garmin_gps.c: use module_usb_serial_driver

This converts the garmin_gps.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: Johan Hovold <jhovold@gmail.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>