wifi: rtw89: use struct to fill H2C command to download beacon frame

Download beacon frame via H2C command for AP mode, and then firmware can
issues beacon periodically. Originally TIM offset minus fixed 24 bytes of
frame header implicitly in macro, and this patch explicitly uses
ieee80211_hdrlen() to get header length, but expected to change nothing
at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240108091325.67424-1-pkshih@realtek.com