power: supply: sbs-battery: Fix a signedness bug in sbs_get_battery_capacity()

The "mode" variable is an enum and in this context GCC treats it as an
unsigned int so the error handling is never triggered.

Fixes: 51d07566 ("bq20z75: Add support for charge properties")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>