[media] allow overriding the driver name

On USB drivers, the dev struct is usually filled with the USB
device. That would mean that the name of the driver specified
by media_device.dev.driver.name would be "usb", instead of the
name of the actual driver that created the media entity.

Add an optional field at the internal struct to allow drivers
to override the driver name.

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