wifi: rtw88: Don't use static local variable in rtw8821c_set_tx_power_index_by_rate

wifi: rtw88: Don't use static local variable in rtw8821c_set_tx_power_index_by_rate

Some users want to plug two identical USB devices at the same time.
This static variable could theoretically cause them to use incorrect
TX power values.

Move the variable to the caller and pass a pointer to it to
rtw8821c_set_tx_power_index_by_rate().

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/fe42858c-9b9f-4f03-9aaa-737472c2cd90@gmail.com