nfp: don't depend on eth_tbl being available

For very very old generation of the management FW Ethernet port
information table may theoretically not be available.  This in
turn will cause the nfp_port structures to not be allocated.

Make sure we don't crash the kernel when there is no eth_tbl:

RIP: 0010:nfp_net_pci_probe+0xf2/0xb40 [nfp]
...
Call Trace:
  nfp_pci_probe+0x6de/0xab0 [nfp]
  local_pci_probe+0x47/0xa0
  work_for_cpu_fn+0x1a/0x30
  process_one_work+0x1de/0x3e0

Found while working with broken/development version of management FW.

Fixes: a5950182 ("nfp: map mac_stats and vf_cfg BARs")
Fixes: 93da7d96 ("nfp: provide nfp_port to of nfp_net_get_mac_addr()")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>