usb: dwc3: omap: use request_threaded_irq()

We intend to share this interrupt with the OTG driver an to ensure
that irqflags match for the shared interrupt handlers we use
request_threaded_irq()

If we don't use request_treaded_irq() then forced threaded irq will
set IRQF_ONESHOT and this won't match with the OTG IRQ handler's
IRQ flags.

NOTE: OTG IRQ handler is yet to be added. This is a preparatory step.

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