ixgbe: Use a signed type to hold error codes

Because error codes are negative, it only makes sense to
consistently use signed types when handling them. Also remove
some explicit comparisons with 0 on these variables.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>