power: bq27xxx_battery: Fix typos and change naming for state of charge functions

Fix typos and change "relative state of charge" to "state of charge" as not
all supported devices use relative state of charge.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>