net/ncsi: Simplify Kconfig/dts control flow

Background:

1. CONFIG_NCSI_OEM_CMD_KEEP_PHY

If this is enabled, we send an extra OEM Intel command in the probe
sequence immediately after discovering a channel (e.g. after "Clear
Initial State").

2. CONFIG_NCSI_OEM_CMD_GET_MAC

If this is enabled, we send one of 3 OEM "Get MAC Address" commands from
Broadcom, Mellanox (Nvidida), and Intel in the *configuration* sequence
for a channel.

3. mellanox,multi-host (or mlx,multi-host)

Introduced by this patch:

https://lore.kernel.org/all/20200108234341.2590674-1-vijaykhemka@fb.com/

Which was actually originally from cosmo.chou@quantatw.com:

https://github.com/facebook/openbmc-linux/commit/9f132a10ec48db84613519258cd8a317fb9c8f1b



Cosmo claimed that the Nvidia ConnectX-4 and ConnectX-6 NIC's don't
respond to Get Version ID, et. al in the probe sequence unless you send
the Set MC Affinity command first.

Problem Statement:

We've been using a combination of #ifdef code blocks and IS_ENABLED()
conditions to conditionally send these OEM commands.

It makes adding any new code around these commands hard to understand.

Solution:

In this patch, I just want to remove the conditionally compiled blocks
of code, and always use IS_ENABLED(...) to do dynamic control flow.

I don't think the small amount of code this adds to non-users of the OEM
Kconfigs is a big deal.

Signed-off-by: Peter Delevoryas <peter@pjd.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>