usb: gadget: r8a66597-udc: add support for TEST_MODE

The USB high speed device must support the TEST_MODE, but the driver
didn't support it. When we sent the SET_FEATURE for TEST_MODE to
the driver, the request was successful, but the module didn't enter
the TEST_MODE.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>