[media] dvb-usb-v2: remove dvb_usb_device NULL check

Reported by Dan Carpenter:

The patch d10d1b9a: "[media] dvb_usb_v2: use dev_* logging
macros" from Jun 26, 2012, leads to the following Smatch complaint:

drivers/media/usb/dvb-usb-v2/dvb_usb_urb.c:31 dvb_usb_v2_generic_io()
	 error: we previously assumed 'd' could be null (see line 29)

...
Remove whole check as it must not happen in any case. Driver is
totally broken if it does not have valid pointer to device.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>