Bluetooth: Controller: Increase default TX buffer count

Only 1 TX buffer is sub-optimal for performance since the controller
would always have to wait for the host TX thread to give it another
buffer. Increase the value to 2 so the controller can keep
transmitting data without waiting for the host.

Change-Id: I4841a6c5010f294996d6fe0fe63260b848a6a437
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>