Bluetooth: Fine-tune default incoming ACL buffer count

In some SMP + GATT scenarios it's possible we get up to 6 incoming ACL
packets. To handle these efficiently, and more importantly, to not
have to drop data if the controller lacks host flow control (as is the
case currently with Zephyr-based controllers), increase the default
from 5 to 6.

For a controller-build it makes sense to just match up with the
controller-side configured RX buffers.

Change-Id: Id44fa724597b88a51f9085dac009e8d84a439bfc
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>