power: supply: bq27xxx_battery: allow kernel poll_interval parameter runtime update

Fix issue with poll_interval being not updated till the previous
interval expired.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Liam Breck <liam@networkimprov.net>
Signed-off-by: Matt Ranostay <matt@ranostay.consulting>
Signed-off-by: Sebastian Reichel <sre@kernel.org>