Bluetooth: common: Kconfig: Extend EVT buf size to store IQ samples

BT_BUF_EVT_RX_SIZE size by default was set to 68 bytes.
In case of build with direction finding connected mode receiver
enabled the size of event buffer was not enough to store all
IQ samples. In such situation IQ samples reports were not delivered
to Host.

The commit changes default size of event RX buffer to 255 in
case the BT_DF_CONNECTION_CTE_RX is enabled.

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