Bluetooth: controller: Refactor Force MD bit implementation

Add Force MD bit feature wherein connection events can be
extended to match the rate at which applications provide
new Tx data. MD bit in Tx PDUs is forced on if there has
been CONFIG_BT_CTLR_TX_BUFFERS count number times Tx/Rx
happened in a connection event. The assumption here is,
if controller's tx buffers are full, it is probable that
the application would send more Tx data during the
connection event, and keeping the connection event alive
will help improve overall throughput.

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