net-thunder: One check less in nicvf_register_interrupts() after error detection

[SF Markus Elfring]

>> Is it a bit more efficient to avoid a double check for the
>> variable "ret" at the end of the current implementation for the
>> discussed function?
> 
> Before asking questions you could answer yourself,
> please look at object code produced by the compiler
> before and after your proposed changes.

* Do any more source code reviewers wonder about the need
  for such a double check?

* Which object code representations would you find representative
  for a further constructive discussion around this
  software component?

Regards,
Markus