korad-kaxxxxp: silence compiler warning (unused value)

korad-kaxxxxp: silence compiler warning (unused value)

Do inspect the return value of the routine which sends the command
before retrieving the response. This silences the following warnings:

../src/hardware/korad-kaxxxxp/protocol.c:179:3: warning: Value stored to 'ret' is never read
                ret = korad_kaxxxxp_send_cmd(serial, "IOUT1?");
../src/hardware/korad-kaxxxxp/protocol.c:184:3: warning: Value stored to 'ret' is never read
                ret = korad_kaxxxxp_send_cmd(serial, "ISET1?");
../src/hardware/korad-kaxxxxp/protocol.c:189:3: warning: Value stored to 'ret' is never read
                ret = korad_kaxxxxp_send_cmd(serial, "VOUT1?");
../src/hardware/korad-kaxxxxp/protocol.c:194:3: warning: Value stored to 'ret' is never read
                ret = korad_kaxxxxp_send_cmd(serial, "VSET1?");
../src/hardware/korad-kaxxxxp/protocol.c:202:3: warning: Value stored to 'ret' is never read
                ret = korad_kaxxxxp_send_cmd(serial, "STATUS?");