rtw88: resolve order of tx power setting routines

Some functions that should be static are unnecessarily exposed, remove
their declaration in header file phy.h.

After resolving their declaration order, they can be declared as static.
So this commit changes nothing except the order and marking them static.

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