USB: serial: keyspan_pda.c: use module_usb_serial_driver

This converts the keyspan_pda.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.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>