sisusb: remove dummy variables

bytes_written parameter of sisusb_copy_memory and sisusb_read_memory
is an out parameter, but its value is never used. So remove it and
pass a dummy variable down to sisusb_read_mem_bulk.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>