firewire: queue the right number of data

There will be 4 padding bytes in struct fw_cdev_event_response on some platforms
The member:__u32 data will point to these padding bytes. While queue the
response and data in complete_transaction in fw-cdev.c, it will queue like this:
|response(excluding padding bytes)|4 padding bytes|4 padding bytes|data.
It queue 4 extra bytes. That is to say it use "&response + sizeof(response)"
while other place of kernel and userspace library use "&response + offsetof
(typeof(response), data)". So it will lost the last 4 bytes of data. This patch
can fix it while not changing the struct definition.

Signed-off-by: JiSheng Zhang <jszhang3@mail.ustc.edu.cn>

This fixes responses to outbound block read requests on 64bit architectures.
Tested on i686, x86-64, and x86-64 with i686 userland, using firecontrol and
gscanbus.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>