ALSA: firewire-lib: Avoid -Wflex-array-member-not-at-end warning

Use the `DEFINE_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:
sound/firewire/amdtp-stream.c:1184:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://github.com/KSPP/linux/issues/202


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Message-ID: <ZgIsBqoMb7p3fMDr@neat>
Signed-off-by: Takashi Iwai <tiwai@suse.de>