[media] cx231xx: use dev_info() for extension load/unload

Now that we're using dev_foo, the logs become like:

	usb 1-2: DVB: registering adapter 0 frontend 0 (Fujitsu mb86A20s)...
	usb 1-2: Successfully loaded cx231xx-dvb
	cx231xx: Cx231xx dvb Extension initialized

It is not clear, by the logs, that usb 1-2 name is an alias for
cx231xx. So, we also need to use dvb_info() at extension load/unload.

After the patch, it will print:
	usb 1-2: Cx231xx dvb Extension initialized

With is coherent with the other logs.

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