Bluetooth: samples/hci_ecc: Switch to using k_thread_spawn()

Switch from fiber_start() to k_thread_spawn(). Also switch to
k_yield() from fiber_yield() in the same go.

Change-Id: I2dc480310f981112063651b61b0ceedf38601bee
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>