nfp: disable netpoll on representors

NFP reprs are software device on top of the PF's vNIC.
The comment above __dev_queue_xmit() sayeth:

 When calling this method, interrupts MUST be enabled.  This is because
 the BH enable code must have IRQs enabled so that it will not deadlock.

For netconsole we can't guarantee IRQ state, let's just
disable netpoll on representors to be on the safe side.

When the initial implementation of NFP reprs was added by the
commit 5de73ee4 ("nfp: general representor implementation")
.ndo_poll_controller was required for netpoll to be enabled.

Fixes: ac3d9dd0 ("netpoll: make ndo_poll_controller() optional")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>