Revert "firewire: Annotate struct fw_iso_packet with __counted_by()"

This reverts commit d3155742.

The header_length field is byte unit, thus it can not express the number of
elements in header field. It seems that the argument for counted_by
attribute can have no arithmetic expression, therefore this commit just
reverts the issued commit.

Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240725161648.130404-1-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>