Ethernet Link instance attribute 6 (Interface Control) shall behave according to the content of instance attribute 11 (Interface Capability)

Today, attribute 6 and 11 have a fix behavior and content as we are interfacing to the netX internal PHYs only. Those are standard Ethernet PHYs and support all 10/100MBit/s and Full/Half Duplex, as well as auto-negotiation.

Therefore, attribute 6 accepts all these settings through a set_attribute_single service and at the same time attribute 11 always shows these settings as supported per default.
Therefore, today attr6 and atte11 are consistent.

However, there are use cases coming up that will add requirements to such attributes:

1. As customer (host application), I want to decide on my own what Ethernet capabilities my product supports.
   This means a customer could decide to support less functionality than the default advertises.
2. As host application, I would like to reflect the capabilities of the (external) PHY used in attribute 11.
   This means that there could be an external PHY connected that has other or limited functionality compared to attribute 11's default.

Implementation:

Adapt the behavior of Ethernet Link instance attribute 6 so that a set_attribute_single service behaves according to the content of attribute 11.

e.g.: if 10MBit/s is not supported according to attribute 11, the set service to attr 6 shall be rejected in case the client wants to write 10MBit/s.

Note: Attribute 11 is already settable by the host application.

Documentation:

Attribute 11 is settable from the host application already today. Therefore, nothing additional needs to be documented.

Test:

• Python TestFramework:
  We need a testcase that walks through all possible attribute 11 combinations and verifies that attr 6 behaves accordingly.
• CT20 Test run for the SPE single port use case:
  Set attr 11 to only support auto-neg and run at least the CT20 Ethernet Link test.