firewire: net: replacing deprecated __attribute__((packed)) with __packed

Fixing a deprecation, replacing __attribute__((packed)) with __packed.
It was deprecated for portability, specifically to avoid GCC specific
code.  See commit 82ddcb04.

Signed-off-by: August Lilleaas <august@augustl.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (added include compiler.h)