Make eeprom reads and writes across block boundary work

Make eeprom reads and writes across block boundary work

Reading and writing across the block boundary was not stepping the
eeprom position after the partial i/o operation at the end of the
first block. This meant that the operation would re-use the end of the
previous block, either re-reading or re-writing it.

Signed-off-by: Keith Packard <keithp@keithp.com>