net: ethtool: don't require empty header nests

Ethtool currently requires a header nest (which is used to carry
the common family options) in all requests including dumps.

  $ cli.py --spec netlink/specs/ethtool.yaml --dump channels-get
  lib.ynl.NlError: Netlink error: Invalid argument
  nl_len = 64 (48) nl_flags = 0x300 nl_type = 2
	error: -22      extack: {'msg': 'request header missing'}

  $ cli.py --spec netlink/specs/ethtool.yaml --dump channels-get \
           --json '{"header":{}}';  )
  [{'combined-count': 1,
    'combined-max': 1,
    'header': {'dev-index': 2, 'dev-name': 'enp1s0'}}]

Requiring the header nest to always be there may seem nice
from the consistency perspective, but it's not serving any
practical purpose. We shouldn't burden the user like this.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>