Bluetooth: settings: Fix missing log_strdup() call

The bt_settings_encode_key() cannot know if the given path pointer is
on the stack or not, so the only safe way to pass it to the logger is
by using a log_strdup() call. Not doing this will likely cause
corrupted strings to show up in the log output.

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