usb:gadget: wait for a while before clean ep0 request

usb:gadget: wait for a while before clean ep0 request

BZ: 37568

after disconnecting pullup, any flying request should be completed or
any potential request should be added within 50ms. 50ms should be enough
considering the interrupt maximun interval. so sleeping for 50ms and
dequeuing ep0 should be able to cleanup the request in ep0.

Change-Id: I9e131cd1217ec748be7a013894ce7c63ef4214a3
Signed-off-by: jzhuan5 <jin.can.zhuang@intel.com>
Reviewed-on: http://android.intel.com:8080/50022
Reviewed-by: Tang, Richard <richard.tang@intel.com>
Reviewed-by: Wu, Hao <hao.wu@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>