b53: Fix build warning.

   drivers/net/dsa/b53/b53_srab.c: In function 'b53_srab_probe':
>> drivers/net/dsa/b53/b53_srab.c:388:20: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
      pdata->chip_id = (u32)of_id->data;
                       ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>