wifi: rtw89: fw: use struct to fill JOIN H2C command

The JOIN command is used to tell firmware an new station is joining, and
create an entry for it. This patch is only to convert to set data via
struct, and don't change logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240115033742.16372-8-pkshih@realtek.com