v4l2codec: fixes for flush() support
v4l2codec: fixes for flush() support

there is no dedicate API for V4l2Codec flush(),
but use STREAMOFF/STREAMON to flush staging buffers.
so, some initialization work needn't twice for STREAMON

zhaobob/libyami,seanvk/libyami,YuJiankang/libyami,YuJiankang/libyami,YuJiankang/libyami,01org/libyami,dspmeng/libyami,stripes416/libyami,01org/libyami,stripes416/libyami,dspmeng/libyami,uartie/libyami,lizhong1008/libyami,zhaobob/libyami,lizhong1008/libyami,stripes416/libyami,01org/libyami,lizhong1008/libyami,seanvk/libyami,dspmeng/libyami,yizhouwei/libyami,yizhouwei/libyami,uartie/libyami,uartie/libyami,zhaobob/libyami,seanvk/libyami,yizhouwei/libyami