mxge(4): avoid clang 21 warning in NO-IP configuration

[dim]

Summary

Building the LINT-NOIP kernel on amd64 with clang 21 results in a
-Werror warning similar to:

sys/dev/mxge/if_mxge.c:1846:44: error: variable 'sum' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
 1846 |                     cksum_offset, sizeof(sum), (caddr_t)&sum);
      |                                                          ^~~

Indeed, if both INET and INET6 are undefined, sum is never
initialized.

I don't know if it makes sense to set a checksum in the TCP header at
all in that configuration, but a simple solution to avoid the warning is
to skip the m_copyback call completely.

Alternatively, sum could be set to zero or another appropriate value,
but I have no idea what is correct here.

[dim]

Initialize sum to zero instead, otherwise we get a "unused variable
'sum'" warning, which also breaks the build.

It's really an esoteric case: INET and INET6 are disabled, and then you
would have to hit a path where TSO is used. Seems very unlikely, and in
that case a zero checksum is probably fine.