[media] media_device: move allocation out of media_device_*_init

Right now, media_device_pci_init and media_device_usb_init does
media_device allocation internaly. That preents its usage when
the media_device struct is embedded on some other structure.

Move memory allocation outside it, to make it more generic.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>