power: supply: core: reduce power_supply_show_usb_type() parameters

Reduce power_supply_show_usb_type() parameter count by folding
power_supply_desc dereference into the function.  This makes following
patch making usb_types const easier.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>