usb: dwc3: dwc3-omap: Disable/Enable only wrapper interrupts in prepare/complete

The dwc3 wrapper driver should not be fiddling with the core interrupts.
Disabling the core interrupts in prepare stops xhci from proper operation.
So remove disable/enable of core interrupts from prepare/complete.

Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>