wifi: rtw89: TX power limit/limit_ru consider negative

Some chips' RF TX power limit/limit_ru tables start to configure some
negative values. Fix the setting logic to prevent negative values from
polluting fields of others.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220812093116.56791-2-pkshih@realtek.com