sensor: bq274xx: re-add function prefixes

Add the bq274xx_ back to the static function prefixes for the bq274xx
driver.

These have been removed recently but every other sensor and most Zephyr
driver have static function prefixed, this helps avoiding ambiguity in
list files, stack traces, setting debugging breakpoints etc.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>