[media] em28xx: restore lost #ifdef

The cleanup that changed the em28xx driver to use v4l2_mc_create_media_graph
instead of its own implementation causes a build error when CONFIG_MEDIA_CONTROLLER
is disabled:

drivers/media/usb/em28xx/em28xx-video.c: In function 'em28xx_v4l2_init':
drivers/media/usb/em28xx/em28xx-video.c:2717:38: error: 'struct em28xx' has no member named 'media_dev'

This puts the new code inside the same #ifdef that controls the presence
of the 'media_dev' member, and that the old code was in.

Fixes: de390787

 ("[media] em2xx: use v4l2_mc_create_media_graph()")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>