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

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. Initialize it to zero to silence the warning.

Reviewed by: jhibbits
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54730