wifi: rtl8xxxu: Fix the TX power of RTL8192CU, RTL8723AU

Don't subtract 1 from the power index. This was added in commit
2fc0b8e5

 ("rtl8xxxu: Add TX power base values for gen1 parts")
for unknown reasons. The vendor drivers don't do this.

Also correct the calculations of values written to
REG_OFDM0_X{C,D}_TX_IQ_IMBALANCE. According to the vendor driver,
these are used for TX power training.

With these changes rtl8xxxu sets the TX power of RTL8192CU the same
as the vendor driver.

None of this appears to have any effect on my RTL8192CU device.

Cc: stable@vger.kernel.org
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/6ae5945b-644e-45e4-a78f-4c7d9c987910@gmail.com