i2c: slave-eeprom: fix boundary check when using sysfs

Due to a copy&paste error, the last byte of the shared memory was not
accessible via sysfs.

Reported-by: Debora Grosse <debora@mds.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>