leds: tca6507: Get rid of duplicate of_node assignment

GPIO library does copy the of_node from the parent device of
the GPIO chip, there is no need to repeat this in the individual
drivers. Remove assignment here.

For the details one may look into the of_gpio_dev_init() implementation.

Call graph:
   --> tca6507_probe_gpios()
     --> gpiochip_add_data()
       --> gpiochip_add_data_with_key()
         --> of_gpio_dev_init()

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>