usb: dwc3: core: don't forget to free coherent memory

commit 3921426b (usb: dwc3: core: move
event buffer allocation out of
dwc3_core_init()) introduced a memory leak
of the coherent memory we use as event
buffers on dwc3 driver.

If the driver is compiled as a dynamically
loadable module and use constantly loads
and unloads the driver, we will continue
to leak the coherent memory allocated during
->probe() because dwc3_free_event_buffers()
is never called during ->remove().

Cc: <stable@vger.kernel.org> # v3.7 v3.8
Signed-off-by: Felipe Balbi <balbi@ti.com>