staging/fwserial: Fold constant MAX_ASYNC_PAYLOAD

Since peer->max_payload is now limited to 1394-2008 spec maximum
of 4096, the port->max_payload limit can now be simplified.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>