usb: dwc3: gadget: Fix BUG in RT config

Using spin_lock() in hard irq handler is pointless
and causes a BUG() in RT (real-time) configuration
so get rid of it.

The reason it's pointless is because the driver is
basically accessing register which is, anyways,
atomic.

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