usb: dwc2: gadget: add TEST_MODE feature support

Handle SET_FEATURE TEST_MODE request sent by the host.
Slightly rework FEATURE request handling to allow parsing
other request types than Endpoint.
Also add a debugfs to change test mode value from user space.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>