Bluetooth: GATT: Fix CCC handling

The 'valid' member of struct bt_gatt_ccc_cfg was redundant, since
setting 'peer' to BT_ADDR_LE_ANY does the same job. What's worse, the
handling of 'valid' was also buggy in that some places looking for
valid CCC structs only matched the address, meaning it might yield a
positive match for invalid entries.

Fix these issues by removing the 'valid' struct member, and solely
using the 'peer' member to identify valid entries. Also simplify the
code by acknowledging that no CCC entry is essentially the same as the
value '0' written to CCC.

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