Compile errors with clang

[uebayasi]

Reported by jsg@:

../../../../dev/ipmi.c:750:2: warning: Value stored to 'v' is never read
        v = bmc_read(sc, _KCS_STATUS_REGISTER);
        ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../dev/ipmi.c:1143:24: warning: The right operand of '+' is a garbage value
        sdrlen = sizeof(shdr) + shdr.record_length;
                              ^ ~~~~~~~~~~~~~~~~~~
../../../../dev/ipmi.c:1814:3: warning: Value stored to 'rc' is never read
                rc = copyout(&ccode, recv->msg.data, 1);      
                ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~