usb: gadget: f_midi: defaults buflen sizes to 512

512 is the value used by wMaxPacketSize, as specified by the USB Spec. This
makes sure this driver uses, by default, the most optimal value for IN and OUT
endpoint requests.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>