usb:langwell_udc: fix NULL pointer in langwell_ep_dequeue

usb:langwell_udc: fix NULL pointer in langwell_ep_dequeue

BZ: 19244

langwell_ep_dequeue calls is_in(ep) which dereferences ep->desc. But ep->desc may be set to NULL due to race condition.
Therefore, need to check ep->desc after seizing the lock again.

Change-Id: I0472fea0de1261c94877d7158ee876760c2686ce
Signed-off-by: jzhuan5 <jin.can.zhuang@intel.com>
Reviewed-on: http://android.intel.com:8080/31511
Reviewed-by: Tang, Richard <richard.tang@intel.com>
Reviewed-by: Kuppuswamy, Sathyanarayanan <sathyanarayanan.kuppuswamy@intel.com>
Tested-by: Sun, Jianhua <jianhua.sun@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>