Fix deep sleep on Inkplate 6 Color

[zachstence]

Expected behavior: Inkplate.setPanelDeepSleepState should work properly

Actual behavior: The following error occurs:

AttributeError: 'int' object has no attribute 'value'

Problem: EPAPER_RST_PIN is an integer constant. The function should be using self.EPAPER_RST_PIN which is a pin. The same applies for other variables used in the function.

Solution: Correctly use the class member variables with self.xxx