firewire: core: avoid -Wflex-array-member-not-at-end warning

firewire: core: avoid -Wflex-array-member-not-at-end warning

Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:

drivers/firewire/core-cdev.c:1141:38: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/Z9NcB81yfPo-8o0h@kspp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>