Some of the power management ops use this style:

  struct device_driver *drv = dev->driver;
  if (drv && drv->pm && drv->pm->prepare(dev))
    drv->pm->prepare(dev);

while others use this:

  const struct dev_pm_ops *pm = dev->driver ? dev->driver->pm : NULL;
  if (pm && pm->runtime_resume)
    pm->runtime_resume(dev);

Convert the first style to the second so they're all consistent.  Remove
local "error" variables when unnecessary.  No functional change intended.

Link: https://lore.kernel.org/r/20191014230016.240912-6-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>