usb: dwc3: gadget: Enable suspend events

commit 72704f87 ("dwc3: gadget: Implement the suspend entry event
handler") introduced (nearly 5 years ago!) an interrupt handler for
U3/L1-L2 suspend events.  The problem is that these events aren't
currently enabled in the DEVTEN register so the handler is never
even invoked.  Fix this simply by enabling the corresponding bit
in dwc3_gadget_enable_irq() using the same revision check as found
in the handler.

Fixes: 72704f87

 ("dwc3: gadget: Implement the suspend entry event handler")
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210428090111.3370-1-jackp@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>