usb: gadget: f_midi: Transmit data only when IN ep is enabled

This makes sure f_midi doesn't try to enqueue data when the IN endpoint is
disabled, ie, USB cable is disconnected.

Reviewed-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>