Fix link fail on x64-64

Weird gcc behavior.  Code snippet:

   uint16_t data;
   char ch;
   ...
   i = (data >> 8) & 0xff;
   ch = data & 0xff;
   ...
   fprintf(smlclog, "%s OTA '01%02o line '%02o set special character %d <%02x>\n", smlctimestamp, device, dc[dx].lineno, i, ch);

Output on x86-64 before this change:

    17:13:51 OTA '0150 line '02 set special character 3 <ffffffff>

Output on armhf or on x86-64 after this change:

    19:05:00.130466808 OTA '0150 line '02 set special character 3 <ff>

WTH?  Masking an unsigned with an appropriately sized mask produces a
sign extension?

devsmlc.h

@@ -250,7 +250,7 @@ int devsmlc (int class, int func, int device) {
   char *bp;
   char buf[SMLC_BUFSIZE];
   int bufidx;
-  char ch;
+  unsigned char ch;
   time_t currenttime;
   uint16_t data;
   ea_t dmcbufbegea;