rtw88: 8723d: implement set_tx_power_index ops

The txagc table is used to map rate_id and txagc register address and
mask, and ops set_tx_power_index uses this table to write TX power to
corresponding registers. Since 8723D is a 1x1 2.4G 11n chip, only CCK, OFDM
and HT_MCS 0-7 are listed in the table.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200422034607.28747-7-yhchuang@realtek.com