ds2760_battery: add rated_capacity module parameter

For systems where the ds2760 is soldered directly on the PCB, the 'rated
capacity' register is not set to anything useful.

In order to allow users to bootstrap this value, introduce a new module
parameter 'rated_capacity' and use it to write the internal EEPROM in
case the value differes from what's been given.

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>