ixgbevf: fix NACK check in ixgbevf_set_uc_addr_vf()

Fix the NACK check in ixgbevf_set_uc_addr_vf() for instances where
index != 0.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>