usb: dwc3: gadget: ignore endpoint IRQs when endpoint is disabled

By the time we're disabling the endpoint, HW
could already have posted more events to our
event buffer. In that case, we will receive
endpoint events for a disabled endpoint.

In order to protect ourselves from that situation,
we simply ignore endpoint interrupts whenever
the endpoint is disabled.

Tested-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>