power: supply: cpcap-charger: Fix charger name

I noticed we have a different name from what Android is using.
Let's not break the /sys/class/power user space interface here
and use the same naming as Android has.

On Android we have the following for droid 4:

$ ls /sys/class/power_supply/
ac battery usb

So let's use the usb naming here for charger too.

Fixes: 0c9888e3 ("power: supply: cpcap-charger: Add minimal
CPCAP PMIC battery charger")
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>