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

In the Linux kernel, the following vulnerability has been resolved:

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.