rtw88: choose the lowest as world-wide power limit

When we are loading tx power limit from the power limit table, compare
the world-wide limit with the current limit and choose the lowest power
limit for the world-wide power settings.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>