gpio: ml-ioh: Cache &pdev->dev to reduce repetition

ioh_gpio_probe() repeats the "&pdev->dev" expression several times.  Cache
the result as "struct device *dev" to reduce the repetition.  No functional
change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>