igc: fix error return handling from call to netif_set_real_num_tx_queues

The call to netif_set_real_num_tx_queues is not assigning the error
return to variable err even though the next line checks err for an
error.  Fix this by adding the missing err assignment.

Detected by CoverityScan, CID#1474551 ("Logically dead code")

Fixes: 3df25e4c ("igc: Add interrupt support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>