power: supply: bq25980: Fix uninitialized wd_reg_val and overrun

Fix the issue when 'i' is equal to array size then array index over
runs the array when checking for the watch dog value.

Fixes: 5069185f ("power: supply: bq25980: Add support for the BQ259xx family")
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>