Suppress unused variable warning in if_mwl.c

With clang 15, the following -Werror warning is produced:

sys/dev/mwl/if_mwl.c:3445:8: error: variable 'ix' set but not used [-Werror,-Wunused-but-set-variable]
        u_int ix;
              ^

Here, 'ix' is a variable that is only used when debugging. Mark the
variable as potentially unused, to suppress the warning.

MFC after: 3 days