usb: dwc3: gadget: re-factor USB2 test mode to a function

There are some situations were we might need to
enable USB Test Modes without having access to a
Host stack. In such situations we cannot rely
solely on USB Control Messages to enable test
features.

For those cases, we will also allow test mode
to be enabled via debugfs and this patch is a
preparation for that.

Signed-off-by: Felipe Balbi <balbi@ti.com>