dev/i2c_eeprom.c: Use negative offset when addr>0xff is mangled with dev addr

On some I2C eeproms which require addresses >1B, the remaining bits are mangled together with the i2c device address. Thus a negative offset_bytes is introduced to cover these cases. The absolute value refers to the amount of mangled bits. E.g. -2 would mean, that the address consists of 10 bits, with two being joined with the i2c device address.