Bluetooth: GATT: Fix assert when storying CCCs

This fixes the following crash:

ASSERTION FAIL [0] @ ZEPHYR_BASE/subsys/logging/log_core.c:180
	argument 2 in log message "%s: Stored CCCs for %s (%s)"
	missing log_strdup().

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>