net: eth: mgmt: Merge 802.1Qav related mgmt requests

There are too many individual requests for Qav related parameters. There
are more Qav parameters that need to be supported (and will be supported
soon - both on the GET and SET side). Handling it the way it was handled
so far would render the eth mgmt API dominated by Qav parameters. That
would make the file hard to read and understand.

Instead of that - use a single GET and SET requests for all Qav
parameters. This works by adding a separate enum with Qav request type
to the ethernet_qav_param struct.

Additionally this approach makes it much easier to document it all since
we now have just a single request and documentation comments in the
ethernet_qav_param struct.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>