power: supply: bq25890: Use the devm_regmap_field_bulk_alloc() helper

Use the devm_regmap_field_bulk_alloc() helper function instead of
open-coding this ourselves.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>