rtw88: unify prefixes for tx power setting routine

Rename the function names to make them have the same prefix "rtw_phy"
for the tx power setting routines. Only the function names and
corresponding identation are modified.

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