serial: 8250_pci: convert to dev_pm_ops

Convert the legacy system PM callbacks to new ones. Meanwhile, remove the
redudant calls to the PCI for changing a power state since it's done by bus
code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>