Bluetooth: Increase RX thread stack size

Normal usage for Bluetooth applications are getting close to or
already overflowing the default BT RX stack size of 1024.

For example:
 - Discovery using the fixed ATT channel used 984 bytes.
 - Discovery using an enhanced ATT channel used 1048 bytes,
   which would lead to stack overflow using the default BT RX thread
   stack size.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>