usb: mtu3: fix kernel panic at qmu transfer done irq handler

usb: mtu3: fix kernel panic at qmu transfer done irq handler

When handle qmu transfer irq, it will unlock @mtu->lock before give back
request, if another thread handle disconnect event at the same time, and
try to disable ep, it may lock @mtu->lock and free qmu ring, then qmu
irq hanlder may get a NULL gpd, avoid the KE by checking gpd's value before
handling it.

e.g.
qmu done irq on cpu0                 thread running on cpu1

qmu_done_tx()
  handle gpd [0]
    mtu3_requ_complete()        mtu3_gadget_ep_disable()
      unlock @mtu->lock
        give back request         lock @mtu->lock
                                    mtu3_ep_disable()
                                      mtu3_gpd_ring_free()
                                   unlock @mtu->lock
      lock @mtu->lock
    get next gpd [1]

[1]: goto [0] to handle next gpd, and next gpd may be NULL.

Fixes: 48e0d3735aa5 ("usb: mtu3: supports new QMU format")
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230417025203.18097-3-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>