power: supply: bq24190_charger: Clean up extcon code

Polishing and fixes for initial extcon patch.

Fixes: 4db249b6f3b4 ("power: supply: bq24190_charger: Use extcon to determine ilimit, 5v boost")
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Breck <kernel@networkimprov.net>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>