rtw88: remove all RTW_MAX_POWER_INDEX macro

Since this macro definition has different values in different chipset,
the current defined macro value is for 8822b. This will cause the
settings of 8822c be incorrect.
Remove RTW_MAX_POWER_INDEX and use max_power_index in struct rtw_chip_info
to make sure the value of different chipset is right.

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>