Fix setting I2C address on ATtinys

[caternuson]

For #93.

Tested on a PID 5743 (gamepad) using following sketch to change I2C address:

#include "Adafruit_seesaw.h"

#define OLD_ADDR 0x50
#define NEW_ADDR 0x55

Adafruit_seesaw ss;

void setup() {
  Serial.begin(9600);
  while (!Serial);

  Serial.println("EEPROM I2C address change test.");

  if (!ss.begin(OLD_ADDR)) {
    Serial.println("seesaw not found.");
    while(1);
  }

  Serial.println("seesaw started.");

  Serial.println("Changing I2C address...");
  ss.setI2CAddr(NEW_ADDR);
  Serial.println("Done.");
}

void loop() {
}

NOTE - I2C breaks after the address change (after call to setI2CAddr())and the attempt to restart it doesn't seem to work. But power cycling works and the device comes up at the newly set I2C address.

[caternuson]

@ladyada for review

[ladyada]

thank you!