ds2760_battery: add current_accum module parameter

When connecting a ds2760 to a partly loaded battery the first time,
there must be a way to bootstrap the current_accum value. Without that,
the current capactity value is bogus until the battery is fully charged
for the first time.

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