staging/fwserial: Limit tx/rx to 1394-2008 spec maximum

Per this conversation https://lkml.org/lkml/2012/11/27/587


limit the maximum transmission to the IEEE 1394-2008 specification
maximum size of 4096 bytes for asynchronous packets.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>