power: supply: bq25890: Fix setting of F_CONV_RATE rate when disabling HiZ mode

The recent "power: supply: bq25890: Add HiZ mode support" change
leaves F_CONV_RATE rate unset when disabling HiZ mode (setting
POWER_SUPPLY_PROP_ONLINE to 1) while a charger is connected.

Separate the resetting HiZ mode (when necessary because of a charger
(re)plug event) into its own "if {}" block which runs first.

And fix the setting of F_CONV_RATE rate by adding helper variables for
the old and new F_CONV_RATE state which check both the online and hiz bits
and then compare the helper variables to see if a F_CONV_RATE update is
necessary.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>