[media] em28xx: support for 2013:0251 PCTV QuatroStick nano (520e)

[media] em28xx: support for 2013:0251 PCTV QuatroStick nano (520e)

Hardware is based of:
Empia EM2884
Micronas DRX 3926K
NXP TDA18271HDC2

... + analog parts.
Analog is not supported currently. Only DVB-T and DVB-C.

There seems to be still	problems for locking DVB-C channels which have
strong signal. Attenuator helps. I think it is demodulator IF/RF AGC
issue. Lets fix it later. Patches are welcome.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>