platform/avr-atmega128rfa1: fixed wrong subregister access to internal random generator

According to [ATmega128RFA1_Datasheet.pdf](http://www.atmel.com/Images/Atmel-8266-MCU_Wireless-ATmega128RFA1_Datasheet.pdf) on page 110 the random values are in bits 5 and 6, not 6 and 7.

Disclaimer: I do not own a system based on atmega128rfa1 platform, but noticed this while experimenting with the rf231 random generator