dwc3-msm: Fix panic issue during unbind of msm_dwc3

During unbind of msm_dwc3, dwc3 core device will be removed due
to which dwc's ipc_log_buf will be freed. Hence remove dbg_event
after removing dwc3 core device as it access ipc_log_buf, which
is already freed to avoid panic.

Change-Id: I7e3d071f454afff4af900f07862327342842679f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>