bq27x00: Prepare code for addition of bq27000 platform driver

This patch simplifies the drivers data structure and moves code to be
shared by the bq27000 and bq27200/bq27500 init functions into a common
function.
This patch has no functional changes, it only moves code around.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>