net: Disable the format-extra-args warning

We will see warnings such as:
net/eth_common.c:57:61: warning: data argument not used by format string [-Wformat-extra-args]
        sprintf(enetvar, index ? "%s%daddr" : "%saddr", base_name, index);
                                              ~~~~~~~~             ^
With clang.  In this case we do not want to re-write our code to be less
compact as the above is intentional and readable.  Add a comment above
the disabling so that it's clear why we want that warning off.

Signed-off-by: Tom Rini <trini@konsulko.com>