dwc3: gadget: Use dbg_event() to log error

dwc3_process_event_entry() API results into calling UART driver
API while trying to print some of error message with USB DWC3
spinlock acquire. In some cases like adb reboot UART driver takes
more time which results into USB DWC3 spinlock bug when ADB thread
context dwc3_gadget_giveback() is being called. Fix this issue
by logging error with USB DWC3 internal buffer using dbg_event()
instead of sending those on serial console.

Change-Id: Idbf3ef1ef939115308e73a841bb0905d4621c9d9
Signed-off-by: Mayank Rana <mrana@codeaurora.org>