igb: fix setting the number of tx queues

The real_num_tx_queues was not being set when in MSI-X only mode.  This patch
corrects that path so all interrupt types are correctly configured.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>