mwl8787: complete cmd waits for error responses

This ensures we don't get 'cmd_wait: timed out' in case we messed
up the command somehow (or if firmware crashes).

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: move ugly u8 casts/len into send_cmd()

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: hack in testmode reply support

This could probably use a bit of cleanup, but it works -- can
now read the reply from the fw in the testmode app.

Signed-off-by: Bob Copeland <me@bobcopeland.com>