Use proper types in kbdmux_kbd_getc():

- The return value should be a signed integer, because -1 means failure.
- The c variable should be unsigned, to force it to be zero-extended
  when returned.

Reported by:	Andreas Tobler <andreast-list fgznet ch>

sys/dev/kbdmux/kbdmux.c

@@ -181,10 +181,10 @@ kbdmux_kbd_putc(kbdmux_state_t *state, char c)
 	state->ks_inq_length++;
 }
 
-static char
+static int
 kbdmux_kbd_getc(kbdmux_state_t *state)
 {
-	char c;
+	unsigned char c;
 
 	if (state->ks_inq_length == 0)
 		return (-1);