extcon: Use dev_of_node(dev) instead of dev->of_node

The dev_of_node function should be preferred.
In the result we may drop unneeded NULL check
of the pointer to the device object.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>