bq27x00: Minor cleanups

* Consistently use uppercase for hexadecimal values.
* Clarify/fix the unit of functions return value in its comment.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>