ixgbe: Fix device ref count bug

The device lookup neglected to do a pci_dev_put() to decrement the
device reference count.

Reported-by: Elena Gurevich <elena.gurevich@toganetworks.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>