power_supply: bq27xxx_battery: Index register numbers by enum

Currently we use tables to map from register function to register number,
these tables assume the enum used to describe the register function
and index the register number is ordered to match the enum order. Index
the register numbers by the enum instead. This also removes the need
to comment each value with its function.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>