usb: gadget: Fix func name_to_index logic for CHARGING function

There is a typo in CHARGING function due to which
func name_to_func_index returns AUDIO_SRC as function index
for CHARGING function. Fix this to allow charger function
enable.

Change-Id: I71ed7addddf49fa46be21dddaadb9a5ce9044da0
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>