Bluetooth: controller: df: Fix check for saturated IQ samples.

When an int16_t I or Q value was input to IQ_CONVERT_12_TO_8_BIT
the compiler would not make the correct comparison with
IQ_SAMPLE_STATURATED_16_BIT causing saturated IQ samples never
being found.

Signed-off-by: Jakob Krantz <jakob.krantz@u-blox.com>