wifi: brcmfmac: Fix field-spanning write in brcmf_scan_params_v2_to_v1()

Using brcmfmac with 6.5-rc3 on a brcmfmac43241b4-sdio triggers
a backtrace caused by the following field-spanning warning:

memcpy: detected field-spanning write (size 120) of single field
  "&params_le->channel_list[0]" at
  drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c:1072 (size 2)

The driver still works after this warning. The warning was introduced by the
new field-spanning write checks which were enabled recently.

Fix this by replacing the channel_list[1] declaration at the end of
the struct with a flexible array declaration.

Most users of struct brcmf_scan_params_le calculate the size to alloc
using the size of the non flex-array part of the struct + needed extra
space, so they do not care about sizeof(struct brcmf_scan_params_le).

brcmf_notify_escan_complete() however uses the struct on the stack,
expecting there to be room for at least 1 entry in the channel-list
to store the special -1 abort channel-id.

To make this work use an anonymous union with a padding member
added + the actual channel_list flexible array.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230729140500.27892-1-hdegoede@redhat.com