bq27x00_battery: Fix flag reading for bq27742

This patch fix the following issues.
- Flag for bq27742 is 2 bytes contracy to 1 byte for older hardware
- Don't read FLAG_CI as bq27742 does not have it
- Use Battery full capacity register as last measure discharge

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>