staging: r8188eu: only OFDM_index_old[0] is used

The odm_TXPowerTrackingCallback_ThermalMeter_8188E function uses only
OFDM_index_old[0]. Use a single variable instead of an array.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220206161809.423031-2-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>