ptmx: When alloc minor, skip the start addr if it has been used

The alloctab can be looped, if the first check the corresponding
bit is set, it will not continue to loop

Signed-off-by: yangsong8 <yangsong8@xiaomi.com>

drivers/serial/ptmx.c

@@ -151,7 +151,7 @@ static int ptmx_minor_allocate(void)
        * prevent (unexpected) infinite loops.
        */
 
-      if (startaddr == minor)
+      if (startaddr == g_ptmx.px_next)
         {
           /* We are back where we started... the are no free device address */