Bluetooth: Remove need for fiber offload

The bt_recv() function had protections for being called from a
preemptible task context, however nothing was protecting preemption by
ISR. A fairly simple fix is to protect the couple of critical regions
that can be reached from bt_recv() with the help of irq_lock().

Change-Id: Ifc29fd31205eb5425e1b7c862347d9420688df4e
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>