serial: 8250_dw: Use an unified new dev variable in probe

Use an unified new dev variable instead of &pdev->dev and p->dev
in probe function.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>