dwc3: Add NULL check before accessing event buffer

Event buffer may not been allocated or configured but software
may tried to deallocate or unconfigure same from EPROBE_DEFER
context which results into NULL pointer dereference. Fix this
issue by adding NULL check before accessing event buffer.

Change-Id: I1d6686e90e54ca17e8ee558f45378ce259eaaf53
Signed-off-by: Mayank Rana <mrana@codeaurora.org>