power: supply: bq25890: Always take HiZ mode into account for ADC rate

The code to check if F_CONV_RATE has been set, or if a manual ADC
conversion needs to be triggered, as well as the code to set
the initial F_CONV_RATE value at probe both where not taking
HiZ mode into account. Add checks for this.

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>