power: bq27x00_battery: Renaming for consistency

Rename functions that are used by multiple devices. New devices
have been added and the function names and driver name are no longer
general enough for the functionality they provide.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: GUAN Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>