media: dvbdev: Fix memory leak in dvb_media_device_free()

In the Linux kernel, the following vulnerability has been resolved: media: dvbdev: Fix memory leak in dvb_media_device_free() dvb_media_device_free() is leaking memory. Free `dvbdev->adapter->conn` before setting it to NULL, as documented in include/media/media-device.h: "The media_entity instance itself must be freed explicitly by the driver if required."