In 04ab5a74f3 a bug was introduced in
the calculation of the GPIO port address by refactoring code. This
fixes the issue by extracting the GPIO port first from the pin.
Moved macros and static inline helper functions needed to access ATmega GPIOs
to cpu/atmega_common/include/atmega_gpio.h in order to reuse them for the
platform specific low level part of the Neopixel driver.