ixgbe: fix for unused variable warning with certain config

If CONFIG_PCI_IOV isn't defined we get an "unused variable" warining so
now wrap the variable declaration like it's usage already was.

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