serial: 8250_dw: Check the data->pclk when get apb_pclk

It should check the data->pclk, not data->clk when get apb_pclk.

Fixes: c8ed99d4("serial: 8250_dw: Add support for deferred probing")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>