Suppress unused variable warning in mfi.c

With clang 15, the following -Werror warnings are produced:

sys/dev/mfi/mfi.c:3698:6: error: variable 'timedout' set but not used [-Werror,-Wunused-but-set-variable]
        int timedout;
            ^
sys/dev/mfi/mfi.c:3742:6: error: variable 'timedout' set but not used [-Werror,-Wunused-but-set-variable]
        int timedout = 0;
            ^

Here, 'timedout' are variables that are only used when debugging,
requiring #if 0 statements to be modified. Mark the variables as
potentially unused, to suppress the warnings.

MFC after: 3 days

(cherry picked from commit 3dbe05f61b65a73582aefdc2ee5a50ad2b4390ef)