[3c595] Fix compilation when "char" is unsigned by default

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/3c595.c

 {       
     int i, j;
     char *reason, *warning;
-    static char prev_conn = -1;
+    static signed char prev_conn = -1;
 
     if (prev_conn == -1) {
         prev_conn = vx_connector;