i40e: always propagate error value in i40e_set_vsi_promisc()

The for loop in i40e_set_vsi_promisc() reports errors via dev_err() but
does not propagate the error up the call chain. Instead it continues the
loop and potentially overwrites the reported error value.
This results in the error being recorded in the log buffer, but the
caller might never know anything went the wrong way.

To avoid this situation i40e_set_vsi_promisc() needs to temporarily store
the error after reporting it. This is still not optimal as multiple
different errors may occur, so store the first error and hope that's
the main issue.

Fixes: 37d318d7 (i40e: Remove scheduling while atomic possibility)
Reported-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>