drivers/nble: Increase RX buffer pool

Since RPC to the Nordic BLE module has no flow control increase
receive buffer pool to handle events from the module. Without this
NBLE stack is not capable of handling all events and we get "No
buffers" error message.

Change-Id: I0566b30a95ef0a027d4533c83c3c2915018a650a
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>