Bluetooth: GATT: Add missing LESC_MASK for encrypt check

The LE Secure connection (LESC) mask also require encryption,
and some users have been using e.g. BT_GATT_PERM_READ_LESC
without BT_GATT_PERM_READ_ENCRYPT, and then the encryption
check in bt_gatt_check_perm was never properly applied.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>