rtw88: fix incorrect tx power limit at 5G

Tx power limit is stored separately by 2G and 5G.
But driver did not get tx power limit from 5G and causes incorrect tx
power. Check if the channel is beyond 2G and get the corresponding tx
power limit.

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