bq27x00_battery: Fixup nominal available capacity reporting

We should not not report nominal available capacity if battery is not
calibrated, as we don't want to represent bogus values in the sysfs and
confuse userland.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>