[media] e4000: implement DC offset correction

[media] e4000: implement DC offset correction

I did some tests against modulator (television signal generator) and this
seems to improve sensitivity a little bit on channel I used. Used device
was able to receive transmission just as weak signal as with Windows XP.
Thanks to Jacek for donating his non-working device. I cannot ask he to
test anymore...

Reported-by: Jacek Konieczny <jajcus@jajcus.net>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>