rtw88: 8821c: add set tx power index

To configure the transmit power of 8821c implement trasmit
power index setting callback function for 8821c. This is very
similar to the callback function of 8822b.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tzu-En Huang <tehuang@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/20200616091625.26489-3-yhchuang@realtek.com