Bluetooth: samples/hci_spi: increase BT_RX_BUF_COUNT to 10

Default BT_RX_BUF_COUNT value is not enough when transfering a large
amount of data (e.g. heavy network traffic over BT 6LoWPAN). Increase
BT_RX_BUF_COUNT from default value 3 to 10.

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>