ixgbe: Fix return value from macvlan filter function

The function to set the macvlan filter should return success or failure
instead of the index of the filter.  The message processing function was
misinterpreting the index as a non-zero return code indicating failure and
NACKing MAC filter set messages that actually succeeded.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>