usb:langwell_ude: Check ep desc to guarantee it is not disabled

usb:langwell_ude: Check ep desc to guarantee it is not disabled

BZ: 20895

A panic is raised in langwell_irq as ep->desc is set to NULL.
In langwell_ep_disable, ep is disabled and spin_lock get unlocked
in langwell_irq to handle the trans complete. In function done,
ep->desc is accessed and we meet NULL pointer. Function done will
be skipped as it is called in nuke to release resources. So no leakage.

Change-Id: I0c72ad83e12b6e61e04128dbafb492417eb1de01
Signed-off-by: feng wang <feng.a.wang@intel.com>
Reviewed-on: http://android.intel.com:8080/33821
Reviewed-by: Zhuang, Jin Can <jin.can.zhuang@intel.com>
Reviewed-by: Tang, Richard <richard.tang@intel.com>
Reviewed-by: Meng, Zhe <zhe.meng@intel.com>
Tested-by: Meng, Zhe <zhe.meng@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>