usb: dwc3: core: move event buffer allocation out of dwc3_core_init()

This patch is in preparation for adding PM support
dwc3 driver. We want to re-use dwc3_core_init and
dwc3_core_exit() functions on resume() and suspend()
callbacks respectively.

Moving even buffer allocation away from dwc3_core_init()
will allow us to reuse the event buffer which was allocated
long ago on our probe() routine.

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