Bluetooth: Add internal function for storing P-192 and P-256 data

Add function to allow adding P-192 and P-256 data to the internal
storage. This also fixes a few coding style issues from the previous
helper functions for the out-of-band credentials storage.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>