Bluetooth: samples: df: Decrease RAM usage in DF connectionless RX

Direction finding connectionless RX sample didn't fit into
nRF52820 SRAM. The sample will not be able to store IQ samples
for maximum number of CTEs allowed by Bluetooth Core 5.3
specification.

For nRF52820 the new allowed maximum number of scanned CTEs
is set to one, to save more space in SRAM for an application.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>