power: supply: bq25890: fix BAT_COMP field definition

According to datasheet, BAT_COMP field spans bits 5-7. The rest of the
code seems to assume this already.

Fixes: 4aeae9cb ("power_supply: Add support for TI BQ25890 charger chip")
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>