olpc-battery: update CHARGE_FULL_DESIGN property for BYD LiFe batteries

Reduce the mAh value for the BYD LiFe battery from 3100mAh to 2800mAh
to better reflect the average usable capacity as measured by olpc-pwr-log.

Signed-off-by: Richard A. Smith <richard@laptop.org>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>