wifi: rtw89: refine add_chan H2C command to encode_bits

Use struct filling style instead of pointer casting.
This does not change the original behavior.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240119081501.25223-2-pkshih@realtek.com