[media] mx2_camera: fix missing unlock on error in mx2_start_streaming()

Add the missing unlock on the error handle path in function
mx2_start_streaming().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>