USB: gadget: android: Do not enable USB if usb_add_config fails

usb_add_config may fail while calling individual function's
bind_configs. This can happen if userspace tries to enable USB
before configuring the functions' parameters properly. Some
examples of such parameters are: DIAG transports and number of
RMNET instances.

Change-Id: I18380f2c6ddf4add0af66a1de62b0c5ae76ac6e4
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>