eth: fbnic: split fbnic_fill()

Factor out handling a single nv from fbnic_fill() to make
it reusable for queue ops.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250901211214.1027927-10-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>