V4L/DVB (3320): Check if PLL-description is set

- Check if in the dvb-usb-device-structure the dvb_pll_desc is set before use.

Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>