firewire: cdev: reduce stack usage by ioctl_dispatch

Replace a hardcoded buffer size by a sizeof union {}.  This shrinks the
stack-allocated ioctl argument buffer from 256 to 40 bytes.  (This is
not much, but subsequent stack usage particularly by the queue_iso ioctl
handler adds up.)

The new form is also easier to keep up to date than a hardcoded size if
more ioctls are added.

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