Bluetooth: Host: Properly handle disable()/enable() cycles

When bt_disable() was introduced, some of the global variables that
handle key aspects of Bluetooth were not converted to be also
re-initialized every time that the stack was disabled and the enabled
again. This meant that the FIFO and semaphore in the device structure
were not reset whenever the corresponding threads using them were shut
down and then restarted.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>