drivers: wifi: siwx91x: Uniformize error management

Usually, the exception are treated in a condition before the nominal
case:

    ret = f();
    if (ret) {
        return -EXXX;
    }
    ... nominal case ...

siwx91x_on_join_ipv4() and siwx91x_on_join_ipv6() didn't follow this
pattern.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>