bq27x00_battery: fix register offset for bq27425

- SOC is at 0x1C so we must add 0x4 as stated in the comment to read the
  right value.
- DCAP is at 0x3c so we also must use a value with the right offset to get
  the correct design capacity.

Actually testing on a bq27410 which has the same register map as bq27425
(but adds new registers).

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>