usb: dwc3: gadget: always enable IOC on bulk/interrupt transfers

by setting IOC always, we can recycle TRBs a
lot sooner at the expense of some increased
CPU load.

The extra load seems to be quite minimal on
OMAP5 devices (instead of 1 IRQ for one MSC
transfer, we get
CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS).

Signed-off-by: Felipe Balbi <balbi@ti.com>