usb: musb: gadget: support musb-specific test modes

we can support the musb-specific test modes on the
vendor specific range of test selector as stated
on USB Specification Table 9-7 Test Mode Selectors.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>