usb: dwc3: core: explicitly setup and cleanup event buffers

Make the call to dwc3_event_buffers_setup()
and dwc3_event_buffers_cleanup() explicit,
so it's easier to implement PM.

Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>