wifi: brcmfmac: fwsignal: Use struct_size() to simplify brcmf_fws_rxreorder()

In the "struct brcmf_ampdu_rx_reorder", change the 'pktslots' field into
flexible array.

It saves the size of a pointer when the memory is allocated and avoids
an indirection when the array is used.
It also removes the usage of a pointer arithmetic and saves a few lines of
code.

Finally, struct_size() can be used. It is not a must have here, because
it is easy to see that buf_size can not overflow, but still, it is a good
practice.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/f4ca6b887ca1290c71e76247218adea4d1c42442.1721547559.git.christophe.jaillet@wanadoo.fr