ds2760_battery: Make charge_now and charge_full writeable

For userspace tools and daemons, it might be necessary to adjust
the charge_now and charge_full properties of the ds2760 battery monitor,
for example for unavoidable corrections due to aging batteries.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Matt Reimer <mreimer@vpop.net>
Cc: Evgeniy Polyakov <zbr@ioremap.net>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alexey Starikovskiy <astarikovskiy@suse.de>
Cc: Len Brown <len.brown@intel.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>