usb: gadget: Add configfs attribuite for controling match_existing_only

Currently the "match_existing_only" of usb_gadget_driver in configfs is
set to one which is not flexible.
Dwc3 udc will be removed when usb core switch to host mode. This causes
failure of writing name of dwc3 udc to configfs's UDC attribuite.
To fix this we need to add a way to change the config of
"match_existing_only".
There are systems like Android do not support udev, so adding
"match_existing_only" attribute to allow configuration by user is cost little.
This patch adds a configfs attribuite for controling match_existing_only
which allow user to config "match_existing_only".

Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Binghui Wang <wangbinghui@hisilicon.com>
Signed-off-by: Yu Chen <chenyu56@huawei.com>