Bluetooth: Controller: Improve default RX/TX buffer counts

Currently the ideal buffer counts for a controller-only build are 6 RX
buffers and 7 TX buffers: ATT_MTU of 158 bytes can be sent in one
connection interval of 6 tx/rx -es wherein connection interval is
7.5ms

Change-Id: I64b4620c5e8e7db8d7ed72fa1db82e266e121f27
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>