ds2781_battery: Use DS2781_PARAM_EEPROM_SIZE and DS2781_USER_EEPROM_SIZE

Since we have defined DS2781_PARAM_EEPROM_SIZE and
DS2781_USER_EEPROM_SIZE, use them to simplify the code.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>