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

The role_maintain H2C command is to align operating mode of WiFi role,
such as STA or AP modes, between driver and firmware.

Use a struct to fill fields of this H2C command.

Don't change logic at all.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250227131228.8457-2-pkshih@realtek.com