sfc: fix XDP queues mode with legacy IRQ

In systems without MSI-X capabilities, xdp_txq_queues_mode is calculated
in efx_allocate_msix_channels, but when enabling MSI-X fails, it was not
changed to a proper default value. This was leading to the driver
thinking that it has dedicated XDP queues, when it didn't.

Fix it by setting xdp_txq_queues_mode to the correct value if the driver
fallbacks to MSI or legacy IRQ mode. The correct value is
EFX_XDP_TX_QUEUES_BORROWED because there are no XDP dedicated queues.

The issue can be easily visible if the kernel is started with pci=nomsi,
then a call trace is shown. It is not shown only with sfc's modparam
interrupt_mode=2. Call trace example:
 WARNING: CPU: 2 PID: 663 at drivers/net/ethernet/sfc/efx_channels.c:828 efx_set_xdp_channels+0x124/0x260 [sfc]
 [...skip...]
 Call Trace:
  <TASK>
  efx_set_channels+0x5c/0xc0 [sfc]
  efx_probe_nic+0x9b/0x15a [sfc]
  efx_probe_all+0x10/0x1a2 [sfc]
  efx_pci_probe_main+0x12/0x156 [sfc]
  efx_pci_probe_post_io+0x18/0x103 [sfc]
  efx_pci_probe.cold+0x154/0x257 [sfc]
  local_pci_probe+0x42/0x80

Fixes: 6215b608

 ("sfc: last resort fallback for lack of xdp tx queues")
Reported-by: Yanghang Liu <yanghliu@redhat.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>