selftest: net: fix typo in altname test

If altname deletion of the short alternative name fails, the error
message printed is: "Failed to add short alternative name".
This is obviously a typo, as we are testing altname deletion.

Fix this using a proper error message.

Fixes: f95e6c9c

 ("selftest: net: add alternative names test")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>