Bluetooth: Reduce the minimum RX buf count to 2

The recent redesign of the H:4 HCI driver means that the smallest
safest RX buffer count goes down to 2. Default is left at 3 however to
get a bit better performance.

Change-Id: I879c7bd3a769f973dfb9bd179d196ab91f9d2abe
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>