samples: Bluetooth: hci_spi: Fix RAM overflow

Fix RAM overflow by using default values for
BT_BUF_ACL_RX_SIZE and BT_BUF_ACL_RX_COUNT. Default Rx size
of 27 is sufficient for nRF51 SoC, as it does not support
data length extensions.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>