wifi: rtw89: fw: consider checksum length of security data

The newer firmware file provides security data with checksum, so we need to
consider the length. Otherwise it will fail to validate total firmware
length resulting in failed to probe.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240204012627.9647-2-pkshih@realtek.com