TimeSync: wrong access rights and default values

The CIP Time Sync object behaves differently in several places from what is described in the API manual.

Access rights:

1. Instance attribute 6 rejects Set Attribute Single with CIP_GSR_BAD_ATTR_DATA for all non-zero values, API manual does not specify any limitations
   --> The attribute behaves correctly. We only allow setting this attribute to value 0 in order to "reset" the "max offset from master". We need to clarify this in the manual.
   
2. Instance attribute 16 and 17 are not settable from host via Set Attributes List, they are rejected with error CIP_GSR_ATTR_NOT_SETTABLE
   --> Instance attribute 16 and 17 (Priorities) are only necessary when being capable of working as a Master-Clock, which we do not and will not support.
   Therefore, those two attributes shall not be supported at all. The host application shall not be able to activate these two attributes.
   Remove theses attribute from the API manual.
   
3. Instance attribute 18 does not apply the new value set via Set Attributes List. Two Object Changes are indicated to the application and the Set Attributes List is returned with success instead.
   --> Must be fixed.
   
4. Default values: The default values for several instance attributes specified in the API manual are not consistent with the implementation. Most values are obtained during Get Attribute from the PTP stack which are mostly not zero.
   The following attributes are affected:
   Attribute 8, 10, 14, 19, 21, 22, 23. Clarify attribute 16, 17 default value "n.a.".
   
   Attr 8,10, 14, 19 , 21, 22, 23: mark default value as "n.a." in API manual
   Attr 16, 17: removed attributes from the API manual.