Update get/set_i2c_address() for ATtiny variants

[caternuson]

This is the same general issue fixed in the Arduino library:
adafruit/Adafruit_Seesaw#93

The EEPROM location used to store the I2C address varies between SAMD and the ATtiny's. Current library is only using a hardwired value for SAMD.

[caternuson]

Fixed with #123.