power_supply: fix up CHARGE_COUNTER output to be more precise

As Richard Smith pointed out, ACR * 6250 / 15 provides for less
precision loss than ACR * 4167 / 10, _and_ it doesn't overflow.  Switch
to using that equation for CHARGE_COUNTER.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: "Richard A. Smith" <richard@laptop.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>