usb: gadget: midi: make FS and HS available

This function works only on FS or HS. If the gadget is HS capable only
HS descriptors are assigned. If we plug it to an 1.1 host it won't work
because we have only 2.0 descriptors. This patch changes the behavior to
provide both sets.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>