ixgbe: fix bug with lots of tx queues

when using more than 8 tx queues you can overrun the 8 bit v_idx
field, so change it to 16 bits to represent the maximum number
of queues (one for each bit)

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>