drivers: fuel_gauge: Fix incorrect variable in bq27xx chem_id switch

The bq27xx driver incorrectly uses the 'val' variable in a switch()
statement that should operate on 'chem_id'. This leads to incorrect
behavior for fuel gauges such as the BQ27427, where the chemical ID
determines how properties are interpreted.

This commit replaces 'switch(val)' with 'switch(chem_id)' to ensure
proper handling of fuel gauge behavior across supported devices.

Signed-off-by: Oleksii Shcherbyna <oleksii.shcherbyna@droid-technologies.com>