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: NFelipe Balbi <balbi@ti.com>