media: ov9734: hold lock to check streaming state

The lock in ov9734 is used to protect the streaming state and
serialize the stream on and off callbacks, it should be hold before
checking the streaming state in ov9734_set_stream().

Reported-by: Tomasz Figa <tfiga@chromium.org>
Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>