filter: HWHDR: simplify flow-control

filter: HWHDR: simplify flow-control

The `interp_mac2str` function concludes with a `switch` followed by a
`return` statement.

The `switch` has one case falling through to a default:

  switch (expr) {
  case X:
    // ... X code ...
  default:
    // ... default code ...
  }

This is equivalent to the simpler and more readily comprehensible:

  if (expr == X) {
    // ... X code ...
  }
  // ... default code ...

Replace the former with the latter.

Doing so makes it obvious that the following `return` statement is never
reached.  Remove it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>