power: supply: ltc2941-battery-gauge: Prepare for LTC2942 and LTC2944

In order to support LTC2942 which has temperature registers
at different offsets than LTC2943 use following register naming
scheme (prefixes):
- LTC294X_ common registers
- LTC2942_ chips specific registers
- LTC2943_ specific registers

LTC2944 is compatible with LTC2943 but uses different constants
to compute voltage and current, so replace num_regs misused
for device indentification with real device id to discriminate
between those two.

There are no functional changes.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Acked-by: Dragos Bogdan <dragos.bogdan@analog.com>
Tested-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>