Bluetooth: samples/peripheral_sc_only: Improve logging

The periphreal_sc_only sample app wasn't very clearly logging
information about pairing failures and could give the false
impression that everything work when it doesn't. Enable some more
logging Kconfig features and take advantage of the pairing complete &
failed callbacks to give more information to the user.

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