Bluetooth: Fix draining nRF51 FIFO during init

It's possible that there's a queued event from the nRF51 at the point
where we start resetting it, resulting in the stack receiving two NOP
events. To prevent this, make sure to drain the UART while the
controller is in a powered off state.

Change-Id: Ic009e11c11ac750fc76c881c0ffe9bd12d38da0a
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>