ds2760_battery: handle full_active_uAh == 0 case correctly

In systems where the battery monitor is not part of the battery pack and
is hence not bootstrapped with sane values, the full_active_uAh is
likely to be zero.

Handle that case by defaulting to the rated_capacity information which
can be passed to the driver using the new module parameter.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Szabolcs Gyurko <szabolcs.gyurko@tlt.hu>
Acked-by: Matt Reimer <mreimer@vpop.net>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>