usb: dwc3: Fix use after free crash from dwc3 interrupt handler

If CONFIG_DEBUG_SHIRQ config option is enabled in kernel, shared
irq interrupt handler can be called from free_irq context and
accessing the freed dwc->ev_buf pointer in irq handler.

Hence fix this issue by explicitly checking for dwc->ev_buf pointer
in dwc3_check_event_buf() function.

Change-Id: I3311aa36408f17fb2560ad02c0929c52e96cc7e8
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>