ixgbe: fix incorrect limit value in ring transverse

We were transversing the tx_ring with IXGBE_NUM_RX_QUEUES.  Now this define
happens to have the correct value but this is misleading and a change later
could easily make this no longer true.  I updated it to netdev->num_tx_queues
like we use in ixgbe_get_strings().

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>