usb: dwc3: skip interrupt when ep disabled

In case EP disabled pass only EPCPLT command
to be handled. In other case we could hit
Bug like below.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000003
IP:
[<ffffffff81673428>] dwc3_thread_interrupt+0x11c8/0x1790

while dep->endpoint.desc is NULL.

Signed-off-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>