ds2760_battery: Fix integer overflow for time_to_empty_now

On the device we're currently developing, battery sizes of ~2.8Ah and
current flow of ~600mA are typical.

With that values, the life_sec computation overflows due to the
multiplication by 3600.

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 <avorontsov@ru.mvista.com>