bq27x00_battery: Do not cache current_now value for bq27000 batery

* This prevent reporting old current_now value for bq27000
* Also ask for current flags, to make sure that current_now
  will be reported with correct signature

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>