i40e/i40evf: fix up type clash in i40e_aq_rc_to_posix conversion

The error code sent into i40e_aq_rc_to_posix() are signed values, so we
really need to treat them as such.

Change-ID: I3d1ae0ee9ae0b1b6f5fc424f8b8cc58b0ea93203
Reported-by: Helin Zhang <helin.zhang@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>