power: ds2781_battery: clean up eeprom read/write functions

The change removes redundant calculation of left space on eeprom while
reading or writing "param_eeprom" or "user_eeprom", the checks are not
needed, since this task is done on caller side in fs/sysfs/file.c

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>