serial: 8250_dw: fix runtime PM initialization sequence

serial: 8250_dw: fix runtime PM initialization sequence

Move pm_runtime_set_active() call earlier in probe to simplify error
handling and add proper error checking to ensure the device is marked
as active before any runtime PM operations can occur.

Additionally, replace the const struct dev_pm_ops declaration with
_DEFINE_DEV_PM_OPS macro for better consistency with modern kernel PM
patterns.

Signed-off-by: Artem Shimko <a.shimko.dev@gmail.com>
Link: https://patch.msgid.link/20251104145433.2316165-3-a.shimko.dev@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>